Abstract - UChicago High Energy Physics

Transcription

Symmetric and Standard Matter-Neutrino Resonances Above Merging Compact
Objects
A. Malkus,1, ∗ G. C. McLaughlin,1, † and R. Surman2, ‡
1
Department of Physics, North Carolina State University, Raleigh, NC 27695 USA.
Department of Physics, University of Notre Dame, Notre Dame, IN 46656 USA.
(Dated: July 6, 2015)
arXiv:1507.00946v1 [hep-ph] 3 Jul 2015
2
Matter-neutrino resonances (MNR) can occur in environments where the flux of electron antineutrinos is greater than the flux of electron neutrinos. These resonances may result in dramatic
neutrino flavor transformation. Compact object merger disks are an example of an environment
where electron antineutrinos outnumber neutrinos. We study MNR resonances in several such disk
configurations and find two qualitatively different types of matter-neutrino resonances: a standard
MNR and a symmetric MNR. We examine the transformation that occurs in each type of resonance
and explore the consequences for nucleosynthesis.
PACS numbers: 14.60.Pq,26.30.Hj
Keywords: neutrino mixing, neutrinos-neutrino interaction, accretion disk
I.
INTRODUCTION
Neutrinos shape the physical phenomena surrounding compact object mergers, from the dynamics of the disk or
hypermassive-neutron star itself [1–4], to the energetic jets, e.g. [5] that may from them. Neutrinos also play an
important role in the nucleosynthesis that takes place in and around disks [6–9]. For example, the wind outflows
[10, 11] above disks can be home to nucleosynthesis, including perhaps the r-process, depending on neutrino flavor
composition [2, 9, 12–16]. The neutrino flavor composition above the neutrino trapping surface depends not only
on thermodynamics in the trapped regions, but also the oscillation of neutrinos as they leave the disk. The high
neutrino density coupled with high matter density provide an environment where several kinds of oscillation may take
place. Neutrinos emitted from mergers can undergo the same types of transformations that neutrinos from supernovae
do [17], as well as oscillations not previously seen elsewhere (except for in collapsars [18]), called Matter-Neutrino
Resonance (MNR) transitions [19]. The MNR takes place when the matter potential and the neutrino self interaction
potential are the same size and have opposite signs.
Much of the previous work on neutrino transformation in high neutrino density environments has considered matter
and self interaction potentials of the same sign. For example, for the case of core collapse supernovae neutrinos, it has
been pointed out that several types of transformations can occur in such systems which have a slight electron neutrino
excess, start at high neutrino density and end at low neutrino density; for recent work see e.g. [20–27]. At very high
densities of neutrinos, synchronized neutrino oscillations, e.g. [28], change neutrino flavor on a small scale. At very
low densities of neutrinos, the neutrino self interaction potential is unimportant and Mikheyev Smirnov Wolfenstein
(MSW) oscillations can take place when the scale of the matter potential is the same as the vacuum scale. In
between these extreme regimes of synchronized and MSW oscillations, large scale flavor transformation can take place
when the neutrino self interaction potential approaches the vacuum oscillation scale [29, 30] and the neutrinos and
antineutrinos enter the transition region nearly in flavor eigenstates, e.g. [29–37]. During the transition, the neutrinos
and antineutrinos are said to be “locked” as their survival probabilities mirror each other and the phenomenon is
referred to as “bipolar” or “nutation” oscillation.
Unique to settings where the neutrino interaction potential and the matter potential have opposite sign, another
oscillation phenomenon can be possible [18, 19]. In [18] it was shown that collapsar-type disks may be home to MNRs
and that the MNR may result in flavor transformation that alters nucleosynthesis. The understanding of the MNR
was expanded in [19], where the standard MNR was explored in detail. A MNR begins when the scale of the matter
potential and the neutrino self interaction potential are the same and can cancel. A MNR transition can cause a
∗ Electronic
address: [email protected]
‡ Electronic address: [email protected]
† Electronic
Diversities in the properties of neutron stars at a
fixed neutron-skin thickness in 208 Pb nucleus
N. Alam1 , A. Sulaksono2, and B. K. Agrawal1∗
1
arXiv:1507.00837v1 [nucl-th] 3 Jul 2015
2
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India
Departemen Fisika, FMIPA, Universitas Indonesia, Depok, 16424, Indonesia.
We study the diversities in the properties of the neutron stars arising due to the different choices for
the cross-coupling between various mesons which governs the density dependence of the nuclear symmetry energy in the extended relativistic mean-field(RMF) model. For this purpose, we obtain two
different families of the extended RMF model corresponding to different non-linear cross-coupling
term in the isovector part of the effective Lagrangian density. The lowest order contributions for the
δ mesons are also included. The different models within the same family are so obtained that they
yield wide variation in the value of neutron-skin thickness in the 208 Pb nucleus. These models are
employed to compute the neutron star properties such as, core-crust transition density, radius and
red shift at canonical mass (1.4M⊙ ), tidal polarizability parameter, and threshold mass required for
the enhanced cooling through direct Urca process. Most of the neutron star properties considered
are significantly different(10%-40%) for the different families of models at a smaller neutron-skin
thickness (∼ 0.15 fm) in the 208 Pb nucleus.
PACS numbers: 21.65.Cd, 21.65.Mn, 21.65.Ef, 26.60.Kp
Keywords:
I.
INTRODUCTION
The neutron stars are believed to be composed of
highly asymmetric matter, predominantly, the neutrons.
Of course, a small admixture of protons, electrons and
muons are also present to maintain the β-equilibrium and
charge neutrality. The various theoretical models have
conjectured the possibility of existence of exotica, like,
hyperons, Bose condensates and quarks in the core of the
neutron stars [1–8]. The precise knowledge of the masses
and the radii of the neutron stars are crucial in determining their compositions. Recently, significant progress
has been made along this direction [9]. The masses of
PSR J1614-2230 [10] and PSR J0348+0432 [11] are measured to be 1.97 ± 0.04M⊙ and 2.01 ± 0.04M⊙, respectively. These measurements impose the lower bounds on
the maximum mass of the neutron stars that a theoretical
model must yield. In the absence of the exotic degrees of
freedoms, the criterion of maximum mass to be ∼ 2M⊙
is readily satisfied by the theoretical models. The observational constraints on the maximum mass of neutron
stars do not completely rule out the possibility of existence of the exotic degrees of freedom, but, the threshold
transition densities for their appearance are pushed to
2.5-3.5 times the nuclear saturation density[12–14]. The
radii of neutron stars are known only poorly. The values
of radii are quite sensitive to the assumed composition
of the atmosphere. The radius R1.4 = 10.7 − 13.1km,
for the neutron star with the canonical mass of 1.4M⊙,
is found to be consistent with the observational analysis
and the host of experimental data for finite nuclei [15].
∗ Electronic
The equations of state (EOSs) for the β-equilibrated
baryonic matter, employed to determine the bulk properties of neutron stars, are usually constructed using the
energy density functionals derived from the Skyrme type
effective forces or from an effective Lagrangian density
associated with the relativistic mean field (RMF) model.
Often, the energy density functionals are optimized using some selected experimental data on a key properties
of the finite nuclei. Occasionally, pseudo-data on nuclear
and neutron matter are also used in the optimization protocols [16–18]. The EOS for the nuclear matter at a given
density and the asymmetry can be viewed for simplicity
as,
ǫ(ρ, I) = ǫ(ρ, 0) + S(ρ)I 2
(1)
where, ǫ(ρ, I) is the energy per nucleon, ρ = ρn + ρp ,
I = (ρn − ρp )/ρ is the asymmetry with ρn and ρp being
densities for the neutrons and the protons. The ǫ(ρ, 0)
is the EOS for the symmetric nuclear matter and S(ρ) is
the symmetry energy coefficient at a density ρ. The EOS
of the symmetric nuclear matter for the densities up to
4.5 times saturation density (ρ0 = 0.16 fm−3 ) is derived
within the reasonable limits by combining the experimental data for the finite nuclei with the collective flow
and kaon production data in heavy-ion collisions [19–21].
The poorly known density dependence of S(ρ) is the major source for uncertainty in the EOS for the asymmetric
matter . The value of S(ρ) is reasonably constrained
only around the saturation density by the bulk properties of the finite nuclei. The understanding of density
dependence of the S(ρ) is crucial as it controls the radii
of neutron stars, the thicknesses of their crusts, the rate
of cooling of neutron stars, and the properties of nuclei
involved in r-process nucleosynthesis. The density dependence of the symmetry energy around the saturation den-
RUP-15-15
Removing Ostrogradski’s ghost from cosmological perturbations
2
2
in f (R, Rµν
, Cµνρσ
) gravity
Yuji Akita1 and Tsutomu Kobayashi1
1
Department of Physics, Rikkyo University, Toshima, Tokyo 175-8501, Japan
arXiv:1507.00812v1 [gr-qc] 3 Jul 2015
Abstract
Recently it was argued that gravity with the squire of the Ricci tensor can be stabilized by adding
constraints to the theory. This was so far demonstrated for fluctuations on the Minkowski/de Sitter
background. We show that the same scheme works equally well for removing Ostrogradski’s ghost
2 , C2
from fluctuations on a cosmological background in generic f (R, Rµν
µνρσ )-type theories of gravity.
We also derive the general formula for the spectrum of primordial tensor perturbations from the
stabilized theory.
PACS numbers: 04.50.Kd
1
Scalar Field Cosmologies With Inverted Potentials
arXiv:1507.00792v1 [gr-qc] 2 Jul 2015
B. Boisseau1∗, H. Giacomini1†, D. Polarski2‡,
1
Université de Tours, Laboratoire de Mathématiques et Physique Théorique,
CNRS/UMR 7350, 37200 Tours, France
2
Université Montpellier & CNRS, Laboratoire Charles Coulomb,
UMR 5221, F-34095 Montpellier, France
July 6, 2015
Abstract
Regular bouncing solutions in the framework of a scalar-tensor gravity model were
found in a recent work. We reconsider the problem in the Einstein frame (EF) in
the present work. Singularities arising at the limit of physical viability of the model
in the Jordan frame (JF) are either of the Big Bang or of the Big Crunch type in
the EF. As a result we obtain integrable scalar field cosmological models in general
relativity (GR) with inverted double-well potentials unbounded from below which
possess solutions regular in the future, tending to a de Sitter space, and starting
with a Big Bang. The existence of the two fixed points for the field dynamics at late
times found earlier in the JF becomes transparent in the EF.
PACS Numbers: 04.62.+v, 98.80.Cq
∗
email:[email protected]
‡
†
1
Hypermagnetic Fields and Baryon Asymmetry from Pseudoscalar Inflation
Mohamed M. Anber∗ and Eray Sabancilar†
Institut de Théorie des Phénomènes Physiques, EPFL, CH-1015 Lausanne, Switzerland.
(Dated: July 6, 2015)
arXiv:1507.00744v1 [hep-th] 2 Jul 2015
We show that maximally helical hypermagnetic fields produced during pseudoscalar inflation can
generate the observed baryon asymmetry of the universe via the B + L anomaly in the Standard
Model. We find that most of the parameter space of pseudoscalar inflation that explains the cosmological data leads to baryon overproduction, hence the models of natural inflation are severely
constrained. We also point out a connection between the baryon number and topology of the relic
magnetic fields. Both the magnitude and sign of magnetic helicity can be detected in future diffuse
gamma ray data. This will be a smoking gun evidence for a link between inflation and the baryon
asymmetry of the Universe.
PACS numbers: 98.80.Cq, 12.15.-y.
Introduction.—Baryon asymmetry of the Universe
(BAU) remains one of the greatest puzzles of cosmology. As was pointed out by Sakharov, any model that
explains BAU has to satisfy three conditions: (1) baryon
number non-conservation, (2) C and CP violations, and
(3) departure from thermal equilibrium [1]. In fact, many
of the early Universe problems were resolved within the
inflationary paradigm. In this regard, one may wonder if
BAU can also find its resolution within inflation.
Since its proposal as a dynamical solution to the strong
CP problem [2], axions provided a playground for rich
phenomenology. These pseudo Nambu-Goldstone bosons
(pseudoscalars) appear as a result of a broken global symmetry. Besides, axion-like particles are abundant in string
theory and are perfect candidates to build UV complete
models of inflation, thanks to their radiatively stable
properties. This stability is attributed to the existence
of a flat direction that is protected by a shift symmetry.
The symmetry eventually gets broken by nonperturbative effects, hence, the desired small values of the slow
roll parameters arise in a technically natural way [3].
Pseudoscalars are naturally coupled to U(1) gauge
fields through the dimension-5 operator (Φ/f )Yµν Y˜ µν ,
which is inevitable from the effective field theory point
of view. This term breaks the conformal invariance of the
theory and leads to the production of cosmologically relevant fields1 [7]. The fields produced via this mechanism
are coherent over the horizon scale and have maximal
helicity [8]. Hence, they carry a non-zero Chern-Simons
density, which breaks the macroscopic CP invariance of
the Universe.
In this letter, we point out that the change in the
˙ proChern-Simons density of the hypercharge field, h,
duced during inflation feeds into the baryon and lepton
number anomaly in the Standard Model, which eventu-
ally gets converted into the BAU. C invariance is broken
since h˙ enters the anomaly equation with opposite signs
for different chiralities (see Eq. (7) and Table I). In addition, h˙ provides the non-equilibrium condition required
by Sakharov’s criteria and sustains the BAU. We find
that natural inflation leads to overproduction of baryons
for most of the parameter space compatible with the cosmological data, hence this class of inflation models are
severely constrained. In what follows, we use the metric
ηµν = diag(1, −1, −1, −1) and set c = 1, ~ = 1, kB = 1.
Generation of the hypercharge field.—We consider a
pseudoscalar inflaton Φ coupled to the U(1)Y hypercharge gauge field Aµ with field strength Yµν :
L=
where V (Φ) is the inflaton potential, f is the axion constant, and α is the dimensionless coupling constant between the axion and hypercharge field. The equation of
motion of the gauge potential Aµ reads2
2
∂
Φ0
2
−
∇
−
α
∇×
A = 0,
(2)
∂τ 2
f
where τ is the conformal time and the prime denotes
the derivative with respect to it.The generation of the
hypercharge field can be envisaged by promoting the
ˆ and then decomposclassical field A to an operator A,
ˆ
ing A into annihilation and creation operators a
ˆkλ as:
R d3 k P
ˆ=
λ Aλ a
ˆkλ eik·x + h.c. , where the cirA
λ=±
(2π)3/2
cular helicity vectors ± obey the relations k · ± = 0
ˆ into (2), we
and k × ± = ∓i|k|± . Substituting A
find that the mode functions A± satisfy the equation
A00± + k 2 ± αkΦ0 /f A± = 0 [9].
2
1
See also Refs. [4–6] for various mechanisms for generating primordial magnetic fields.
1
1
α
(∂µ Φ)2 − V (Φ) − Yµν Y µν −
ΦYµν Y˜ µν , (1)
2
4
4f
Here, we use the radiation gauge setting A0 = 0 and ∇ · A = 0.
In this gauge, the electric and magnetic fields are E = −A0 /a2
and B = ∇ × A/a2 , where a is the scale factor in the FriedmanRobertson-Walker Universe.
Mon. Not. R. Astron. Soc. 000, 1–5 (2015)
Printed 6 July 2015
(MN LATEX style file v2.2)
arXiv:1507.00992v1 [astro-ph.CO] 3 Jul 2015
Astrophysical constraints on massive black hole binary evolution
from Pulsar Timing Arrays
Hannah Middleton,1? Walter Del Pozzo,1 Will M. Farr,1 Alberto Sesana1
and
Alberto Vecchio1
1
School of Physics & Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
Accepted . . . Received . . . ; in original form . . .
ABSTRACT
We consider massive black hole binary systems and information that can be derived about
their population and formation history solely from current and possible future pulsar timing
array (PTA) results. We use models of the stochastic gravitational-wave background from
circular massive black hole binaries with chirp mass in the range 106 − 1011 M evolving
solely due to radiation reaction. Our parameterised models for the black hole merger history
make only weak assumptions about the properties of the black holes merging over cosmic
time. We show that current PTA results place a model-independent upper limit on the merger
density of massive black hole binaries, but provide no information about their redshift or mass
distribution. We show that even in the case of a detection resulting from a factor of 10 increase
in amplitude sensitivity, PTAs will only put weak constraints on the source merger density as a
function of mass, and will not provide any additional information on the redshift distribution.
Without additional assumptions or information from other observations, a detection cannot
meaningfully bound the massive black hole merger rate above zero for any particular mass.
Key words: gravitational waves, massive black holes, pulsar timing arrays
1
INTRODUCTION
Massive black holes (MBHs) reside at the centre of most galaxies
(see e.g. Kormendy & Ho 2013, and references therein), and are
believed to have a central role in their evolution (see e.g. Volonteri
2012, and references therein for a recent review). Mapping the
population of MBHs, studying their properties, demographics, and
their connection to the broader formation of structure is one of
the open problems of modern astrophysics. It is however difficult
to tackle, due to the large range of scales and the wide variety of
physical processes involved. The MBH evolutionary path remains
a highly debated subject with many competing hypotheses still in
play. Currently favoured hierarchical structure formation scenarios
imply frequent galaxy mergers (White & Rees 1978). As a result
MBH binaries (MBHBs) should be quite common in the Universe
(Begelman et al. 1980; Volonteri et al. 2003). To-date there is no
confirmed observed MBHB, although a number of candidates exist
(see e.g. Dotti et al. 2012, and references therein) and tantalising
claims have been recently made (Graham et al. 2015; Liu et al.
2015).
A means to survey MBHBs is through the observation of gravitational waves (GWs) that these systems generate as they inspiral
towards their final merger. The accurate timing of an array of highlystable millisecond pulsars – a Pulsar Timing Array (PTA, Foster
?
E-mail: [email protected]
& Backer 1990) – provides a direct observational means to probe
the cosmic population of MBHBs on orbital timescales of order
of several years. Astrophysical modelling suggests that the radiation emitted by an ensemble of MBHBs produces a GW stochastic
background in the frequency range ∼ 10−9 − 10−7 Hz, where PTAs
operate (Sesana et al. 2008, 2009; Ravi et al. 2012; Sesana 2013b).
Such a background affects the time of arrival of pulses in a characteristic fashion (Sazhin 1978; Detweiler 1979; Hellings & Downs
1983), which can be used to discriminate the signal from a plethora
of other undesired effects (Lentati et al. 2015).
Over the last decade pulsar timing has been used to put progressively tighter constraints on gravitational radiation in this frequency
regime, (see, e.g. Jenet et al. 2006). More recently the three international consortia of the Parkes Pulsar Timing Array (Shannon et al.
2013), NANOGrav (Demorest et al. 2013) and European Pulsar
Timing Array (Lentati et al. 2015; van Haasteren et al. 2012, 2011)
have used data from observations of unprecedented sensitivity to
place constraints that are starting to probe astrophysically interesting
regions of the parameter space (Sesana 2013b; Shannon et al. 2013).
In this paper, we consider a GW stochastic background produced by MBHBs in circular orbits losing energy and angular momentum purely through GW emission. We use an analytical merger
rate model which makes minimal assumptions about the cosmological history of MBHB evolution and can capture the key characteristics of simulation results to investigate the astrophysical implications
of current (Shannon et al. 2013; Lentati et al. 2015), and future plau-
Dark matter velocity dispersion effects on CMB and
matter power spectra
O. F. Piattella,a∗ L. Casarini,a† J. C. Fabris,a‡
and J. A. de Freitas Pachecoa,b§
a Department of Physics, Universidade Federal do Esp´ırito Santo,
avenida Ferrari 514, 29075-910 Vitória, Esp´ırito Santo, Brazil
b Université de Nice-Sophia Antipolis, Observatoire de la Côte d’Azur,
Laboratoire Lagrange, Nice Cedex 4, France
July 6, 2015
Abstract
Effects of velocity dispersion of dark matter particles on the CMB TT power
spectrum and on the matter linear power spectrum are investigated using a
modified CAMB code. Cold dark matter originated from thermal equilibrium
processes does not produce appreciable effects but this is not the case if particles
have a non-thermal origin. A cut-off in the matter power spectrum at small
scales, similar to that produced by warm dark matter or that produced in
the late forming dark matter scenario, appears as a consequence of velocity
dispersion effects, which acts as a pressure perturbation.
∗
[email protected]
[email protected]
‡
[email protected]
§
[email protected]
†
1
arXiv:1507.00935v1 [astro-ph.HE] 3 Jul 2015
Astrophysics
Gamma-ray emission from binaries in context
Guillaume Dubus
Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
CNRS, IPAG, F-38000 Grenoble, France
Abstract
More than a dozen binary systems are now established as sources of variable, high energy (HE, 0.1 − 100 GeV)
gamma rays. Five are also established sources of very high energy (VHE, > 100 GeV) gamma rays. The mechanisms
behind gamma-ray emission in binaries are very diverse. My current understanding is that they divide up into
four types of systems: gamma-ray binaries, powered by pulsar rotation; microquasars, powered by accretion onto
a black hole or neutron star; novae, powered by thermonuclear runaway on a white dwarf; colliding wind binaries,
powered by stellar winds from massive stars. Some of these types had long been suspected to emit gamma rays
(microquasars), others have taken the community by surprise (novae). My purpose here is to provide a brief
review of the current status of gamma-ray emission from binaries, in the context of related objects where similar
mechanisms are at work (pulsar wind nebulae, active galactic nuclei, supernova remnants).
Résumé
Emission gamma des systèmes binaires. Plus d’une douzaine de systèmes binaires sont maintenant identifiés
comme des sources variables de rayonnement gamma de haute énergie (0.1 − 100 GeV). Cinq systèmes binaires
sont également des sources de rayonnement gamma de très haute énergie (> 100 GeV). Les processus menant à
l’émission de ce rayonnement sont variés. On peut néanmoins diviser ces systèmes en quatre grandes classes : les
binaires gamma, dont le moteur est la rotation d’un pulsar ; les microquasars, dont le moteur est l’accrétion sur
un trou noir ou une étoile à neutrons ; les novae, dont l’énergie provient de la combustion nucléaire à la surface
d’une naine blanche ; les binaires à collision de vents, qui dissipent l’énergie cinétique de vents d’étoiles massives.
On soupçonnait depuis longtemps que certaines classes de systèmes binaires devaient être associées à de l’émission
gamma (les microquasars) ; pour d’autres classes, la découverte d’émission gamma a été une surprise (les novae).
Je propose ici une brève revue de nos connaissances sur l’émission gamma de haute et très haute énergie dans
les systèmes binaires, avec le souci de souligner les liens avec d’autres objets astrophysiques dans lesquels des
processus similaires sont à l’oeuvre (nébuleuses de pulsar, noyaux actifs de galaxie, restes de supernova).
Keywords : Acceleration of particles ; Radiation mechanisms : non-thermal ; Binaries : general ; Gamma-rays : stars
Mots-clés : Accélération de particules ; mécanismes de rayonnement non-thermiques ; étoiles binaires ; rayonnement gamma : étoiles
Email address: [email protected] (Guillaume Dubus).
Preprint submitted to Elsevier Science
July 6, 2015
Prepared for submission to JCAP
On stars, galaxies and black holes in massive bigravity
Jonas Enandera,b Edvard M¨
ortsella,b
a
Oskar Klein Centre, Stockholm University,
Albanova University Center
106 91 Stockholm, Sweden
b
Department of Physics, Stockholm University
AlbaNova University Center
106 91 Stockholm, Sweden
E-mail: [email protected], [email protected]
Abstract: In this paper we study the phenomenology of stars and galaxies in massive
bigravity. We give parameter conditions for the existence of viable star solutions when the
radius of the star is much smaller than the Compton wavelength of the graviton. If these
parameter conditions are not met, we constrain the ratio between the coupling constants of
the two metrics, in order to give viable conditions for e.g. neutron stars. For galaxies, we
put constraints on both the Compton wavelength of the graviton and the conformal factor
and coupling constants of the two metrics. The relationship between black holes and stars,
and whether the former can be formed from the latter, is discussed. We argue that the
different asymptotic structure of stars and black holes makes it unlikely that black holes
form from the gravitational collapse of stars in massive bigravity.
Keywords: modified gravity, bigravity, massive gravity, stars, galaxies, black holes, Vainshtein mechanism
Deep photometry of galaxies in the VEGAS survey: the case
of NGC 4472
arXiv:1507.00911v1 [astro-ph.GA] 3 Jul 2015
Marilena Spavone on behalf of the VEGAS team⋆
Abstract The VST-VEGAS project is aimed at
observing and studying a rich sample of nearby
early-type galaxies in order to systematically characterize their properties over a wide baseline of
sizes and out to the faint outskirts where data are
rather scarce so far. The external regions of galaxies more easily retain signatures about the formation and evolution mechanisms which shaped
them, as their relaxation time are longer, and they
are more weakly influenced by processes such as
mergers, secular evolution, central black hole activity, and supernova feedback on the ISM, which
tend to level age and metallicity gradients. The
collection of a wide photometric dataset of a large
number of galaxies in various environmental conditions, may help to shed light on these questions.
To this end VEGAS exploits the potential of the
VLT Survey Telescope (VST) which provides high
quality images of one square degree field of view
in order to satisfy both the requirement of high
⋆ VEGAS team: M. Capaccioli (P.I.), Michele Cantiello,
D. A. Forbes, A. Grado, E. Iodice, L. Limatola, N. Napolitano, M. Paolillo, T. H. Puzia, G. Raimondo, A. J. Romanowsky, P. Schipani & M. Spavone
resolution data and the need of studying nearby,
and thus large, objects. We present a detailed study
of the surface photometry of the elliptical galaxy
NGC4472 and of smaller ETGs in its field, performed by using new g and i bands images to constrain the formation history of this nearby giant
galaxy, and to investigate the presence of very faint
substructures in its surroundings.
1 The VEGAS survey
The VST Elliptical GAlaxies Survey (VEGAS) is
a deep multi-band (g, r, i) imaging survey of earlytype galaxies in the Southern hemisphere carried
out with VST at the ESO Cerro Paranal Observatory (Chile). The survey goal is to map the
surface brightness of galaxies with Vrad < 4000
km/s, sampling all environmental conditions and
the whole parameter space. The expected depths
at S/N > 3 in the g, r and i bands are 27.3, 26.8,
and 26 mag arcsec−2 respectively, enough to detect signatures of diffuse star components around
1
Mon. Not. R. Astron. Soc. 000, 1–?? (2015)
Printed 6 July 2015
Origin of the high vlos feature in the Galactic bar
Michael Aumer
?
and Ralph Schönrich
Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford, OX1 3RP, UK
submitted on June 19, 2015
ABSTRACT
We analyse a controlled N -body + smoothed particle hydrodynamics simulation
of a growing disc galaxy within a non-growing, live dark halo. The disc is continuously
fed with gas and star particles on near-circular orbits and develops a bar comparable
in size to the one of the Milky Way (MW). We extract line of sight velocity vlos distributions from the model and compare it to data recently obtained from the APOGEE
survey which show distinct high velocity features around vlos ∼ 200 km s−1 . With an
APOGEE like selection function, but without any scaling nor adjustment, we find vlos
distributions very similar to those in APOGEE. The stars that make up the high vlos
features at positive longitudes l are preferentially young bar stars (age τ . 2 − 3 Gyr)
which move away from us along the rear side of the bar. At negative l, we find the
corresponding low vlos feature from stars moving towards us. At l > 10 degrees the
highest vlos stars are a mixture of bar and background disc stars which complicates
the interpretation of observations. The main peak in vlos is dominated by fore- and
background stars. At a given time, ∼ 40 − 50 per cent of high vlos stars occupy x1
like orbits, but a significant fraction are on more complex orbits. The observed feature
is likely due to a population of dynamically cool, young stars formed from gas just
outside the bar and subsequently captured by the growing bar. The high vlos features
disappear at high latitudes |b| & 2 degrees which explains the non-detection of such
features in other surveys.
Key words: Galaxy: bulge; methods: numerical - Galaxy: evolution - Galaxy: kinematics and dynamics - Galaxy: structure;
1
INTRODUCTION
The central region of our Galaxy is dominated by a bar
(Blitz & Spergel 1991; Binney et al. 1991), which consists
of an X-shaped box/peanut bulge at R < 2 kpc (Wegg &
Gerhard 2013) and a vertically thin part, the long bar, extending to R ∼ 4 − 5 kpc (Wegg et al. 2015). Recently, the
Apache Point Observatory Galactic Evolution Experiment
(APOGEE) survey (Allende Prieto et al. 2008) has revealed
that the line of sight velocity vlos distributions of ∼ 4 700
K/M giant stars towards the Galactic centre at Galactic coordinates l = 4 − 14 degrees and |b| 6 2 degrees exhibit distinct shoulders at high velocities vlos ∼ 200 km s−1 (Nidever
et al. 2012) . This result was followed up by other surveys,
which yielded differing results: On the one hand, Babusiaux et al. (2014) analysed the vlos distribution of ∼ 400 bar
red clump stars at l = (−6) − 10 degrees and |b| 6 1 degrees and found hints of a distinct high vlos component at
vlos > 200 km s−1 at positive l and additionally of a distinct
low vlos component at vlos < −200 km s−1 at negative l. On
?
E-mail:[email protected] (MA)
the other hand, Zoccali et al. (2014) failed to detect distinct high or low vlos components in a survey of ∼ 5 000 red
clump stars in the central Galaxy. However, they were probing different sight-lines, most of them at |b| > 2 degrees and
thus further away from the plane than the other authors, or
pointing more closely towards the Galactic centre at l = 0.
Bars are rotating triaxial structures which are expected
to be supported mostly by stars on prograde orbits parented
by the x1 family of closed long-axis orbits (Contopoulos &
Papayannopoulos 1980). These orbits have proven essential
in explaining the (l, v) diagram of gas towards the Galactic
centre (Binney et al. 1991) and are thus expected to be an
important ingredient for explaining any observation of the
kinematics of bar stars. Molloy et al. (2015) have recently
shown that they could recover a distinct high vlos peak in an
N -body simulation of a barred galaxy if they analysed only
2:1 resonant orbits elongated along the long bar axis, i.e. x1
orbits. They did not recover the feature when taking into
account all stars. Previously, Nidever et al. (2012) and Li et
al. (2014) had failed to recover the observational feature in
N -body simulations of Milky Way (MW) like galaxies. This
led Li et al. (2014) to conclude that the high vlos peak is
IO:I: A Near-Infrared Camera for the Liverpool Telescope
R. M. Barnsleya , H. E. Jermaka , I. A. Steelea , R. J. Smitha , S. D. Batesa , C. J. Mottrama
arXiv:1507.00906v1 [astro-ph.IM] 3 Jul 2015
a Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool, UK, L3 5RF
Abstract. IO:I is a new instrument that has recently been commissioned for the Liverpool Telescope, extending
current imaging capabilities beyond the optical and into the near infrared. Cost has been minimised by use of a previously decommissioned instrument’s cryostat as the base for a prototype and retrofitting it with Teledyne’s 1.7µm cutoff
Hawaii-2RG HgCdTe detector, SIDECAR ASIC controller and JADE2 interface card. In this paper, the mechanical,
electronic and cryogenic aspects of the cryostat retrofitting process will be reviewed together with a description of the
software/hardware setup. This is followed by a discussion of the results derived from characterisation tests, including
measurements of read noise, conversion gain, full well depth and linearity. The paper closes with a brief overview of
the autonomous data reduction process and the presentation of results from photometric testing conducted on on-sky,
pipeline processed data.
Keywords: HgCdTe, IR Detectors, Hawaii-2RG, SIDECAR ASIC, JADE2, Liverpool Telescope, Imaging, Infrared.
Address all correspondence to: Rob Barnsley, Astrophysics Research Institute, Liverpool John Moores University,
146 Brownlow Hill, Liverpool, UK, L3 5RF; Tel: +44 151 231 2934; E-mail: [email protected]
1 Introduction
The Liverpool Telescope1 (LT) is a fully robotic 2m telescope situated on the Canary Island of La
Palma. It is an unmanned facility insomuch that it does not require an observer; all observations
are fed into an autonomous scheduler. The scheduler provides the telescope with a ranked list of
suitable observing programmes based on current and predicted future meteorological conditions,
programme priority and other queue optimisation conditions. This functionality makes the LT
particularly well suited to time-domain observational astronomy where its ability for rapid response
has been a key asset.
The LT is a multi-purpose facility, and as such is equipped with a suite of instruments including
a 10′ x10′ optical imager (IO:O), an integral-field spectrograph2, 3 (FRODOSpec), a fast-readout
camera (RISE) and a polarimeter4 (RINGO3), all of which are available for use on any single night.
1
Printed 6 July 2015
Pulsar lensing geometry
Siqi
Liu1,3? , Ue-Li Pen1,2 †, J-P Macquart4 ‡, Walter Brisken5 §, Adam Deller6 ¶
1
2
arXiv:1507.00884v1 [astro-ph.SR] 3 Jul 2015
3
4
5
6
Canadian Institute for Theoretical Astrophysics, University of Toronto, M5S 3H8 Ontario, Canada
Canadian Institute for Advanced Research, Program in Cosmology and Gravitation
Department of Astronomy and Astrophysics, University of Toronto, M5S 3H4, Ontario, Canada
ICRAR-Curtin University of Technology, Department of Imaging and Applied Physics, GPO Box U1978, Perth, Western Australia 6102, USA
National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801, USA
ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
6 July 2015
ABSTRACT
Our analysis of archival VLBI data of PSR 0834+06 revealed that its scintillation
properties can be precisely modelled using the inclined sheet model (Pen & Levin
2014), resulting in two distinct lens planes. These data strongly favour the grazing
sheet model over turbulence as the primary source of pulsar scattering. This model
can reproduce the parameters of the observed diffractive scintillation with an accuracy at the percent level. Comparison with new VLBI proper motion results in a direct
measure of the ionized ISM screen transverse velocity. The results are consistent with
ISM velocities local to the PSR 0834+06 sight-line (through the Galaxy). The simple
1-D structure of the lenses opens up the possibility of using interstellar lenses as precision probes for pulsar lens mapping, precision transverse motions in the ISM, and new
opportunities for removing scattering to improve pulsar timing. We describe the parameters and observables of this double screen system. While relative screen distances
can in principle be accurately determined, a global conformal distance degeneracy
exists that allows a rescaling of the absolute distance scale. For PSR B0834+06, we
present VLBI astrometry results that provide (for the first time) a direct measurement
of the distance of the pulsar. For targets where independent distance measurements
are not available, which are the cases for most of the recycled millisecond pulsars that
are the targets of precision timing observations, the degeneracy presented in the lens
modelling could be broken if the pulsar resides in a binary system.
Key words: Pulsars: individual (B0834+06) – scattering – waves – magnetic: reconnection – techniques: interferometric – ISM: structure
1
INTRODUCTION
Pulsars have long provided a rich source of astrophysical information due to their compact emission and predictable
timing. One of the least well constrained parameters for
most pulsars is their distance. For some pulsars, timing parallax or VLBI parallax has resulted in direct distance determination. For most pulsars, the distance is a major uncertainty for precision timing interpretations, including mass,
moment of inertia (Kramer et al. 2006; Lorimer & Kramer
2012), and gravitational wave direction (Boyle & Pen 2012).
Direct VLBI observations of PSR B0834+06 show mul?
†
‡
§
¶
Email: [email protected]
tiple images lensed by the interstellar plasma. Combining
the angular positions and scintillation delays, the authors
(Brisken et al. 2010) (hereafter B10) published the derived
effective distance (defined in Section 2.1) of 1168 ± 23 pc for
apexes on the main scattering axis. This represents a precise
measurement compared to all other attempts to derive distances to this pulsar. This effective distance is a combination
of pulsar-screen and earth-screen distances, and does not allow a separate determination of the individual distances. A
binary pulsar system would in principle allow a breaking
of this degeneracy (Pen & Levin 2014). One potential limitation is the precision to which the lensing model can be
understood.
In this paper, we examine the geometric nature of the
lensing screens. In B10, VLBI astrometric mapping directly
demonstrated the highly collinear nature of a single dominant lensing structure. First hints of single plane collinear
Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear
flows
X. Fang, C. Xia, R. Keppens and T. Van Doorsselaere
Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven,
Celestijnenlaan 200B, 3001 Leuven, Belgium
ABSTRACT
We extend our earlier multidimensional, magnetohydrodynamic simulations of coronal rain occurring in magnetic arcades with higher resolution, grid-adaptive computations covering a much longer (> 6 hour) timespan. We quantify how in-situ forming
blob-like condensations grow along and across field lines and show that rain showers can
occur in limit cycles, here demonstrated for the first time in 2.5D setups. We discuss
dynamical, multi-dimensional aspects of the rebound shocks generated by the siphon
inflows and quantify the thermodynamics of a prominence-corona-transition-region like
structure surrounding the blobs. We point out the correlation between condensation
rates and the cross-sectional size of loop systems where catastrophic cooling takes place.
We also study the variations of the typical number density, kinetic energy and temperature while blobs descend, impact and sink into the transition region. In addition, we
explain the mechanisms leading to concurrent upflows while the blobs descend. As a
result, there are plenty of shear flows generated with relative velocity difference around
80 km s−1 in our simulations. These shear flows are siphon flows set up by multiple blob
dynamics and they in turn affect the deformation of the falling blobs. In particular, we
show how shear flows can break apart blobs into smaller fragments, within minutes.
Subject headings: magnetohydrodynamics(MHD) — Sun: corona — Sun: filaments,
prominences
1.
Introduction
Observations show recurrent formation and reshuffling of cool and dense material in coronal
loops. The small scale (O(100) km) coronal rain is observed as cold, dense elongated blob-like
features condensing in a much hotter loop and descending along one of its legs. The rain is
guided by the loop magnetic field (Beckers 1962), dropping from heights of tens of Mm into the
chromosphere (Kawaguchi 1970; Leroy 1972). Similar phenomena have been observed by analysing
absorption profiles in EUV spectral lines (Schrijver 2001; O’Shea et al. 2007). Seen to propagate
from the top of the loop towards its footpoints (De Groof et al. 2004), systematic intensity variations
in EUV spectral lines are confirmed as downflows of cool plasma, rather than representing slow
July 6, 2015
A Study of Active Shielding Optimized for
1–80 keV Wide-Band X-ray Detector in Space
Yoshihiro Furuta1 , Yuki Murota1 , Junko S. Hiraga2 ,
Makoto Sasano1 , Hiroaki Murakami1 , and Kazuhiro Nakazawa1
1
Department of Physics, Graduate School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
Department of Physics, School of Science and Technology,
Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
Active shielding is an effective technique to reduce background signals
in hard X-ray detectors and to enable observing darker sources with high
sensitivity in space. Usually the main detector is covered with some shield
detectors made of scintillator crystals such as BGO (Bi4 Ge3 O12 ), and the
background signals are filtered out using anti-coincidence among them.
Japanese X-ray observing satellites “Suzaku” and “ASTRO-H ” employed
this technique in their hard X-ray instruments observing at > 10 keV.
In the next generation X-ray satellites, such as the NGHXT proposal,
a single hybrid detector is expected to cover both soft (1–10 keV) and hard
(> 10 keV) X-rays for effectiveness. However, present active shielding is
not optimized for the soft X-ray band, 1–10 keV. For example, Bi and
Ge, which are contained in BGO, have their fluorescence emission lines
around 10 keV. These lines appear in the background spectra obtained by
ASTRO-H Hard X-ray Imager, which are non-negligible in its observation
energy band of 5–80 keV.
We are now optimizing the design of active shields for both soft and
hard X-rays at the same time. As a first step, we utilized a BGO crystal
as a default material, and measured the L lines of Bi and K lines of Ge
from it using the X-ray SOIPIX, “XRPIX”.
PRESENTED AT
International Workshop on SOI Pixel
Detector (SOIPIX2015), Tohoku University,
Sendai, Miyagi, Japan, June 3–6, 2015
1
1.1
Introduction
Active Shielding Used in X-ray Detectors
X-ray observation of celestial objects tells us much about the high-energy phenomena
in the universe. In the hard X-ray band (> 10 keV), which is a sensitivity frontier,
reduction of detector-originated background is crucial for accurate measurements,
because the number of photons from the objects gets smaller.
Active shielding is a technique widely used to reduce backgrounds significantly
in hard X-ray detectors. The main detector is covered with another larger one, and
cosmic-ray originated and scattered photon events are excluded using anti-coincidence
between them. Japanese X-ray observing satellites “Suzaku” and “ASTRO-H ” employed this technique in their hard X-ray instruments [1–3]. Figure 1 shows ASTRO-H
and the Hard X-ray Imager (HXI) onboard it, which adopts BGO (Bi4 Ge3 O12 ) scintillators as its active shields.
Figure 1: (Left) Whole view of the ASTRO-H satellite to be launched at the end of
Japanese fiscal year 2015. At the tail of the satellite, marked by a red line, are the
two HXIs (Hard X-ray Imagers). (Right) Cross section view of the HXI, where the
main detector is marked by a red line and the BGO shields are colored in blue.
1.2
Issues of Active Shielding at the Soft X-ray Band
BGO is one of the most suitable materials for active shielding. It has a large photoabsorption cross section because of the high atomic number of Bi (Z = 83) and its
high density (ρ = 7.13 g/cm−3 ). Additionally, it is not deliquescent and is easy to get
large ingots. There is a possibility, however, that the characteristic X-ray fluorescent
lines of Bi and Ge around 10 keV become nuisances in the soft X-ray (1–10 keV)
1
Draft version July 6, 2015
Preprint typeset using LATEX style emulateapj v. 5/2/11
HYDRODYNAMIC PROPERTIES OF GAMMA-RAY BURSTS OUTFLOWS DEDUCED FROM THERMAL
COMPONENT
Asaf Pe’er 1 , Hugh Barlow 1 , Shane O’Mahony 1 , Raffaella Margutti 2 , Felix Ryde 3 , Josefin Larsson 3 ,
Davide Lazzatti 4 , Mario Livio 5
ABSTRACT
We study the properties of a significant thermal emission component that was identified in 47 GRBs
observed by different instruments. Within the framework of the “fireball” model, we deduce the values
of the Lorentz factor Γ, and the acceleration radius, r0 , for these bursts. We find that all the values of
Γ in our sample are in the range 102 ≤ Γ ≤ 103 , with hΓi = 310. We find a very weak dependence of
Γ on the acceleration radius r0 , Γ ∝ r0α with α = −0.10 ± 0.09 at σ = 2.1 confidence level. The values
of r0 span a wide range, 107 ≤ r0 ≤ 109.5 cm, with mean value hr0 i ∼ 108.5 cm. This is higher than
the gravitational radius of a 10M⊙ black hole by a factor ≈ 100. We argue that this result provides
indirect evidence for jet propagation inside a massive star, and suggests the existence of recollimation
shocks that take place close to this radius.
Subject headings: gamma-rays:bursts — hydrodynamics — methods: data analysis — radiation mechanisms:thermal — radiative transfer
1. INTRODUCTION
One of the major developments in the study of gammaray bursts (GRBs) in recent years has been the realization that a thermal component may be a key spectral
ingredient. While the shape of most GRB spectra do not
resemble a “Planck” function, in a non-negligible minority of GRBs a careful spectral analysis reveals a spectral component that is consistent with having a blackbody (“Planck”) shape, accompanied by an additional,
non-thermal part. Following pioneering work by Ryde
(2004, 2005), such a component was clearly identified in
56 GRBs detected by BATSE (Ryde & Pe’er 2009) and
is now identified in several Fermi GRBs as well. A few
notable examples are GRB090902B (Ryde et al. 2010),
GRB100507 (Ghirlanda et al. 2013), GRB100724B
(Guiriec et al. 2011), GRB110721A (Axelsson et al.
2012; Iyyani et al. 2013), GRB120323A (Guiriec et al.
2013) and GRB101219B (Larsson et al. 2015).6
The existence of a thermal emission component may
hold the key to understanding the prompt emission spectra. First, this component provides a physical explanation to at least part of the observed spectra. Furthermore, thermal photons serve as seed photons for inverse
Compton (IC) scattering by energetic electrons, therefore
these photons may play an important role in explaining
1
2
Physics Department, University College Cork, Cork, Ireland
Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA
3 The Oskar Klein Centre for Cosmoparticle Physics, SE-106
91 Stockholm, Sweden; Department of Physics, KTH Royal Institute of Technology, AlbaNova, SE-106 91 Stockholm, Sweden
4 Department of Physics, Oregon State University, 301
Weniger Hall, Corvallis, OR 97331, USA
5 Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA
6 A long lasting thermal component was identified in addition in several low luminosity GRBs, such as GRB060218
(Campana et al. 2006) and GRB100316D (Starling et al. 2011,
2012). As these GRBs may have a different origin than “classical” GRBs (Shcherbakov et al. 2013; Margutti et al. 2013), they
are omitted from the analysis carried out here.
the non-thermal part of the spectra as well. In fact, as
it was recently shown by Axelsson & Borgonovo (2015),
while clear “Planck” spectra are only rarely observed,
the narrowness of the spectral width of GRBs rules out
a pure synchrotron origin in nearly 100% of the GRB
spectra observed to date. Thus, it is possible that modified “Planck” spectra contribute to the observed emission
in a very large fraction of GRBs.
Thermal photons decouple from the plasma at the photosphere, which is by definition the inner most region
from which an electromagnetic signal can reach the observer. Therefore the properties of a Plank spectral component directly reveal the physical conditions at the photosphere, in those GRBs in which it can be directly identified. This feature is in contrast to the non-thermal
spectral component, the origin of which is still uncertain. This is due to the fact that the radiative origin
of the non-thermal component is still debatable, and its
exact emission radius is very poorly constrained, both
theoretically and observationally.
In the framework of the classical “fireball” model
(Paczynski 1986; Rees & Meszaros 1992, 1994), the photospheric radius, rph depends only on two free model
parameters: the luminosity, L, and the Lorentz factor, Γ at the photospheric radius7 (e.g., Paczynski
1990; Abramowicz et al. 1991; Mész´
aros & Rees 2000;
Mész´
aros 2006, and references therein). The observed
temperature weakly depends, in addition, on a third pa1/6
rameter, r0 , as T ob ∝ r0 . Here, r0 is the acceleration radius, which is the radius where the acceleration of plasma
to relativistic (kinetic) motion begins. Thus, by definition, at this radius the bulk Lorentz factor Γ(r0 ) = 1,
while at larger radii Γ(r) increases at the expense of the
internal energy. In the classical “fireball” model, where
the outflow expands freely and magnetic fields are sub7 This statement holds under the assumption that the photospheric radius rph is larger than the saturation radius, rsat , which
is the radius at which all the available internal energy is converted
to kinetic energy.
Over Saturation in SiPMs:
The Difference Between Signal Charge and Signal Amplitude
Max L. Ahnen1,
ETH Zurich, Institute for Particle Physics, CH-8093 Zurich, Switzerland
Abstract
A recent report on the over saturation in SiPMs is puzzling. The measurements, using a variety of SiPMs, show an
excess in signal far beyond the physical limit of the number of SiPM microcells without indication of an ultimate
saturation. In this work I propose a solution to this problem. Different measurements and theoretical models of
avalanche propagation indicate that multiple simultaneous primary avalanches produce an ever narrower and faster
signal. This is because of a speed-up of effective avalanche propagation processes. It means that SiPMs, operated at
their saturation regime, should become faster the more light they detect. Therefore, signal extraction methods that use
the amplitude of the signal should see an over saturation effect. Measurements with a commercial SiPM illuminated
with bright picosecond pulses in the saturation regime demonstrate that indeed the rising edge of the SiPM signal
gets faster as the light pulses get brighter. A signal extractor based on the amplitude shows a nonlinear behavior
in comparison to an integrating charge extractor. This supports the proposed solution for the over saturation effect.
Furthermore I show that this effect can already be seen with a bandwidth of 300MHz, which means that it should be
taken into account for fast sampling experiments.
Keywords: Over Saturation, Signal Extraction, Avalanche Breakdown, SiPM, GAPD, Geiger Avalanche Diode
1. Introduction
Silicon Photomiltipliers (SiPMs) are novel photo detectors designed to detect weak light signals and are
used for a large variety of applications. Each SiPM consists of a matrix of avalanche diode cells with a common
readout. The absorbed photons produce hole-electron
pairs in the depleted region. Then the carriers are accelerated in the reverse bias electric field and ionize
more carriers in an avalanche breakdown process. The
avalanche diodes are operated in the Geiger regime.
Recently, Gruber et al. [1] reported on the over saturation in SiPMs. Their measurements showed an increase
in signal beyond the physical limit of the number of
SiPM microcells without indication of an ultimate saturation. They speculated that simultaneous avalanches
could induce a higher charge signal. Here, however, I
want to propose a physically more plausible model.
Theoretical models of avalanche propagation [2] suggest that multiple simultaneous primary avalanches produce an ever narrower and faster microcell signal. This
Email address: [email protected] (Max L. Ahnen)
Preprint submitted to
results from the speed-up of effective diffusion processes. When a photon triggers an avalanche in an SiPM
microcell, it grows vertically and laterally and reaches
a maximum lateral size of about 10µm after less than a
nanosecond. This size is usually smaller than the microcell size [3], Therefore the time resolution depends
on where in the SiPM microcell the avalanche was triggered [4]. Popova et al. [2] indeed demonstrate that
single microcells produce a narrower, higher amplitude
pulse when exposed to bright light flashes. Nevertheless
they observe that the charge within the pulses stays the
same. Such a behavior can be understood if the charge
released is constant and the microcell gets faster, see
Fig. 1.
Furthermore it means that the entire SiPM detecting
bright, saturating sub-nanosecond light flashes should
become intrinsically faster the more light it detects. This
is because every single microcells get faster the more
simultaneous photons they are exposed to.
This suggests that charge signal extractors should
see normal saturation, while amplitude signal extractors should see over saturation — a signal beyond the
number of physical microcells. Gruber et al. reported
July 6, 2015
c ESO 2015
Astronomy & Astrophysics manuscript no. 25296˙ap˙edited
July 6, 2015
Mass distributions of star clusters for different
star formation histories in a galaxy cluster environment
C. Schulz1 , J. Pflamm-Altenburg2 , and P. Kroupa2
1
European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany
Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), Universität Bonn, Nussallee 14 - 16, D-53115 Bonn, Germany
e-mail: [email protected], [jpflamm;pavel]@astro.uni-bonn.de
2
July 6, 2015
ABSTRACT
Clusters of galaxies usually contain rich populations of globular clusters (GCs). We investigate how different star formation histories
(SFHs) shape the final mass distribution of star clusters.
We assumed that every star cluster population forms during a formation epoch of length δt at a constant star-formation rate (SFR).
The mass distribution of such a population is described by the embedded cluster mass function (ECMF), which is a pure power law
extending to an upper limit Mmax . Since the SFR determines Mmax , the ECMF implicitly depends on the SFR.
Starting with different SFHs, the time-evolution of the SFR, each SFH is divided into formation epochs of length δt at different
SFRs. The requested mass function arises from the superposition of the star clusters of all formation epochs. An improved optimal
sampling technique is introduced that allows generating number and mass distributions, both of which accurately agree with the
ECMF. Moreover, for each SFH the distribution function of all involved SFRs, F(SFR), is computed. For monotonically decreasing
SFHs, we found that F(SFR) always follows a power law.
With F(SFR), we developed the theory of the integrated galactic embedded cluster mass function (IGECMF). The latter describes
the distribution function of birth stellar masses of star clusters that accumulated over a formation episode much longer than δt. The
IGECMF indeed reproduces the mass distribution of star clusters created according to the superposition principle. Interestingly, all
considered SFHs lead to a turn-down with increasing star cluster mass in their respective IGECMFs in a similar way as is observed
for GC systems in different galaxy clusters, which offers the possibility of determining the conditions under which a GC system was
assembled.
Although assuming a pure power-law ECMF, a Schechter-like IGECMF emerges from the superposition principle. In the past decade,
a turn-down at the high-mass end has been observed in the cluster initial mass function. This turn-down can be explained naturally if
the observed star cluster ensembles are superpositions of several individual star cluster populations that formed at different times at
different SFRs.
Key words. galaxies: clusters: general – galaxies: star clusters: general – methods: analytical
1. Introduction
Galaxy clusters form through the coalescence of galaxy groups
and the infall of individual galaxies. In the course of time, the
number of galaxies in a galaxy cluster steadily grows, thereby
increasing the density of galaxies and enlarging the probability
for galaxy-galaxy encounters during which star formation takes
place. Thus, galaxy clusters and their major galaxies were assembled from many such encounters in which new stars and star
clusters (SCs) were formed, as well as through the accretion of
gas, stars, and SCs.
During such collisions, star formation is expected to occur at
a higher rate than observed in today’s Universe since the early
galaxies were notably gas-rich. In these events, SCs of a wide
mass range were formed, and due to the high star-formation
rates (SFRs), a substantial number of high-mass SCs were able
to form (e.g., Larsen & Richtler 2000). Even if massive stars
have short lifetimes and low-mass SCs dissolve fast in a tidal
field, at least the high-mass SCs have a considerable chance to
survive over a Hubble time, enabling us to observe them today
as globular clusters (GCs).
Our idea is to use the surrounding SCs – in this case, the
ancient GCs that probably formed during the above-mentioned
interactions – to derive the star formation activities at that time
and to determine under which conditions the major galaxies, and
in the end, the host galaxy cluster itself, were assembled. The
overall mass distribution of these GCs may show features that
bear a memory of such events because in galaxy-galaxy interactions SC populations are formed during bursty phases, while
infalling galaxies contribute SC populations that largely formed
under quiescent conditions. Other collaborations also use SCs to
constrain properties of the host galaxy (e.g., Côté et al. 1998;
Maschberger & Kroupa 2007; Beasley et al. 2008; Norris &
Kannappan 2011; Georgiev et al. 2012). To this end, it is investigated how the star formation activities influence the mass
distribution of SCs to provide the theoretical groundwork for analyzing SC systems in the future.
What shapes the mass distribution of SCs? Since the major
galaxies in a galaxy cluster have probably undergone several interaction processes with intense star formation episodes in the
past, an SC sample observed around such a galaxy is most likely
a superposition of different SC populations formed at different
times. From SC formation it is known that the mass distribution
function of a newly born SC population that formed coevally
can be described by the so-called embedded cluster mass function (ECMF) (e.g., Lada & Lada 2003; Kroupa & Weidner 2003;
Weidner & Kroupa 2005). We here assumed that SC formation
1
c ESO 2015
Astronomy & Astrophysics manuscript no. 25545˙hk˙LF˙final
July 6, 2015
Letter to the Editor
The broad-band radio spectrum of LS I +61◦ 303 in outburst
L. Zimmermann, L. Fuhrmann? and M. Massi
Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received 2014;
ABSTRACT
Aims. Our aim is to explore the broad-band radio continuum spectrum of LS I +61◦ 303 during its outbursts by employing the available
set of secondary focus receivers of the Effelsberg 100 m telescope.
Methods. The clear periodicity of the system LS I +61◦ 303 allowed observations to be scheduled covering the large radio outburst in
March-April 2012. We observed LS I +61◦ 303 on 14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32 GHz with a cadence
of about 12 hours followed by two additional observations several days later. Based on these observations we obtained a total of 24
quasi-simultaneous broad-band radio spectra.
Results. During onset, the main flare shows an almost flat broad-band spectrum, most prominently seen on March 27, 2012, where
– for the first time – a flat spectrum (α = 0.00 ± 0.07, S ∝ να ) is observed up to 32 GHz (9 mm wavelength). The flare decay phase
shows superimposed “sub-flares” with the spectral index oscillating between −0.4 and −0.1 in a quasi-regular fashion. Finally, the
spectral index steepens during the decay phase, showing optically thin emission with values α ∼ −0.5 to −0.7.
Conclusions. The radio characteristics of LS I +61◦ 303 compare well with those of the microquasars XTE J1752-223 and Cygnus X3. In these systems the flaring phase is actually also composed of a sequence of outbursts with clearly different spectral characteristics:
a first outburst with a flat/inverted spectrum followed by a bursting phase of optically thin emission.
Key words. Radio continuum: stars - Galaxies: jets - X-rays: binaries - X-rays: individual (LS I +61◦ 303 ) - Gamma-rays: stars
1. Introduction
The periodical radio emitting stellar system LS I +61 303 consists of a fast rotating Be star and a compact object of unclear
nature. One hypothesis is that the compact object is a radio pulsar and that the relativistic electrons responsible for the radio
outburst are produced through an interaction between the relativistic wind of the pulsar and the wind of the Be star (Maraschi
& Treves 1981; Dhawan et al. 2006; Dubus 2006), i.e. similar to
the system PSR B1259−63. This system is formed by a O9.5Ve
star and a compact object with well-detected radio pulses, i.e.
a radio pulsar. The radio outburst of PSR B1259−63 occurring around periastron passage is explained by electrons accelerated at the shock front between the two winds (Chernyakova
et al. 2014, and references therein). For the compact object in
LS I +61◦ 303 with the radio outburst towards apastron, however, an alternative hypothesis suggests an accreting object, i.e. a
black hole or a neutron star (e.g. Taylor et al. 1992; Romero et al.
2007; Massi & Kaufman Bernadó 2009). In this case, the radio
emitting electrons belong to a jet typical of microquasars. The
Bondi & Hoyle (1944) accretion theory for the eccentric orbit
of LS I +61◦ 303 with an orbital period P =26.496 days (phase
Φ) then predicts two accretion maxima and therefore two ejections of relativistic electrons (Taylor et al. 1992; Marti & Paredes
1995; Bosch-Ramon et al. 2006; Romero et al. 2007). During the
first ejection near periastron (Φperi = 0.23 ± 0.02, Casares et al.
2005) relativistic electrons suffer severe inverse-Compton (IC)
losses due to the proximity to the Be star. Consequently, a high
energy outburst is expected, as indeed confirmed by Fermi/LAT
observations (Abdo et al. 2009), but no or only negligible radio
emission (Bosch-Ramon et al. 2006). For the second accretion
◦
?
corresponding author: [email protected]
peak near apastron it has been demonstrated by Bosch-Ramon
et al. (2006) that the larger distance between the compact object
and the Be star results in lower IC losses (i.e. a smaller high energy outburst) and the relativistic electrons can propagate out of
the orbital plane into a radio jet. Recent Fermi/LAT observations
also confirmed this second, smaller high energy outburst (Jaron
& Massi 2014).
Synchrotron radiation emitted from one relativistic electron
population with density Nrel gyrating along a magnetic field B
produces a power-law radio spectrum (S ∝ να ) with spectral
slope αthin < 0 above a critical frequency νbreak that depends on
B and Nrel . Below this critical frequency self-absorption effects
become important and the emission becomes optically thick with
a spectral slope αthick = 2.5 (e.g. Kaiser 2006). For instance,
the spectrum of the radio outburst in the pulsar system PSR
B1259−63 has a spectral index α = −0.7, which is consistent with optically thin synchrotron emission (Fig. 3 in Connors
et al. 2002). In contrast, the jets of microquasars often show
flat/inverted radio spectra as demonstrated by Fender (2001).
Plasma and magnetic field variations along the jet in fact may
create regions with different B and Nrel values resulting in spectral components with different turn-over frequencies νbreak . The
overlap of the optically thin part of one spectral component with
the self-absorbed part of the adjacent one will result in a flat
spectrum when observed with a spatial resolution insufficient to
resolve the jet (Kaiser 2006). In the microquasar Cygnus X-1,
radio emission has been measured with a flat spectrum, i.e. α =
−0.06 ± 0.05 and α = 0.07 ± 0.04 (Fender et al. 2000). In Cygnus
X-1 the black hole is powered by accretion of the stellar wind of
its supergiant companion star. Since the companion is close to
filling its Roche lobe, the wind is not symmetric but strongly
focused towards the black hole (Miˇskoviˇcová et al. 2011, and
1
High energy signatures of quasi-spherical accretion onto rotating,
magnetized neutron star in the ejector-accretor intermediate state
W. Bednarek and P. Banasiński
Department of Astrophysics, University of Łód´z, ul. Pomorska 149/153, 90-236 Łód´z, Poland
Abstract
We consider a simple scenario for the accretion of matter onto a neutron star in order to understand processes in the inner pulsar magnetosphere during the transition stage between different
accretion modes. A simple quasi-spherical accretion process onto rotating, magnetized compact object is analysed in order to search for the radiative signatures which could appear during
transition between ejecting and accreting modes. It is argued that different accretion modes can
be present in a single neutron star along different magnetic field lines for specific range of parameters characterising the pulsar (rotational period, surface magnetic field strength) and the
density of surrounding medium. The radiation processes characteristic for the ejecting pulsar,
i.e. curvature and synchrotron radiation produced by primary electrons in the pulsar outer gap,
are expected to be modified by the presence of additional thermal radiation from the neutron star
surface. We predict that during the transition from the pure ejector to the pure accretor mode
(or vice versa) an intermediate accretion state can be distinguished which is characterised by the
γ-ray spectra of pulsars truncated below ∼ 1 GeV due to the absorption of synchro/curvature
spectrum produced in the pulsar gaps.
Keywords: binaries: general — pulsars: general — accretion — radiation mechanisms:
non-thermal — gamma-rays: stars
1.
2. Introduction
The importance of the accretion of matter onto compact objects as a key process for generation
of energy around compact objects has been recognized since early 70-ties. In fact, three basic
modes for the interaction of compact object with the surrounding matter have been distinguished,
i.e. the ejector, accretor and propeller modes. These different models have been observed and
investigated in the broad energy range. It is expected that neutron stars (NSs) can change their accretion modes with the evolution of their parameters (rotational period, magnetic field strength)
or with the change of the parameters of the surrounding medium. In fact, the external conditions
around ejecting pulsars can change significantly when they enter from time to time dense clouds
in the interstellar medium. The surrounding plasma, attracted by strong gravitational field of
the neutron star, can overcome the pressure of the pulsar wind initializing the transition from
the ejecting to the accreting pulsar. Similar transition process can also happen in the opposite
direction when the accreting neutron star emerges from a dense cloud into a rare interstellar
Preprint submitted to Journal of High Energy Astrophysics
July 6, 2015
c ESO 2015
Astronomy & Astrophysics manuscript no. WOarxiv
July 6, 2015
Massive stars on the verge of exploding: the properties of oxygen
sequence Wolf-Rayet stars?
F. Tramper1 , S. M. Straal1,2 , D. Sanyal3 , H. Sana4 , A. de Koter1,5 , G. Gräfener6 , N. Langer3 , J. S. Vink6 ,
S. E. de Mink1 , and L. Kaper1
1
2
3
4
5
6
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE
Amsterdam, The Netherlands
ASTRON, The Netherlands Institute for Radio Astronomy, PO Box 2, 7790 AA Dwingeloo, The Netherlands
Argelander Institut f¨
ur Astronomie, University of Bonn, Auf dem H¨
ugel 71, D-53121, Bonn, Germany
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
Armagh Observatory, College Hill, BT61 9DG Armagh, UK
Preprint online version: July 6, 2015
ABSTRACT
Context. Oxygen sequence Wolf-Rayet (WO) stars represent a very rare stage in the evolution of massive stars. Their
spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The
properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive
stars, as well as their remaining lifetime and the expected properties of their supernovae.
Aims. We aim to homogeneously analyse the currently known presumed-single WO stars to obtain the key stellar and
outflow properties and to constrain their evolutionary state.
Methods. We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter
spectra of the WO stars and deduce the atmospheric parameters. We calculate dedicated evolutionary models to
determine the evolutionary state of the stars.
Results. The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface
helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models
with helium zero-age main sequence masses of MHe,ini = 15 − 25M that exhibit a fairly strong (on the order of a few
times 10−5 M yr−1 ), homogeneous (fc > 0.3) stellar wind.
Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of Mini = 40 − 60M .
They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.
1. Introduction
The enigmatic oxygen sequence Wolf-Rayet (WO) stars
represent a very rare stage in massive star evolution. Their
spectra are characterized by strong emission lines of highly
ionized carbon and oxygen, and in particular a strong O vi
λ3811-34 ˚
A emission line. Their emission-line spectra point
to dense, outflowing atmospheres. Despite their rarity, the
WO stars can provide key information in our understanding
of massive star evolution.
Although the progenitors of hydrogen-free type Ib and
Ic supernovae (SNe) have not yet been identified, it is very
likely that they are evolved Wolf-Rayet stars (e.g., Yoon
et al. 2012). In particular, WO stars are potential progenitors of the helium-deficient type Ic SNe, as they may have
a very low helium abundance. If they are rapidly rotating
when they explode, they could produce an associated longduration gamma-ray burst (e.g., Woosley & Bloom 2006).
The tell-tale signature of WO stars is their O vi λ381134 ˚
A emission. This emission was first found in the spectra of the central stars of planetary nebulae. However,
Sanduleak (1971) pointed out that five of the stars showing
?
Based on observations obtained at the European Southern
Observatory under program IDs 091.C-0934 and 093.D-0591.
O vi λ3811-34 ˚
A have broad, Wolf-Rayet like emission lines
and do not appear to have an associated nebula. It was
therefore suggested that they are part of the carbon sequence Wolf-Rayet (WC) class. Barlow & Hummer (1982)
argued that these WC O vi stars should be seen as a separate class of Wolf-Rayet stars, and introduced the first WO
classification scheme.
Since their discovery, the WO class has been commonly
interpreted as a very short stage of evolution of massive stars covering an initial mass range of approximately
45 − 60M after the carbon sequence Wolf-Rayet (WC)
phase (e.g., Sander et al. 2012; Langer 2012). In such a
scenario, the emission lines of highly ionized oxygen reflect
the high oxygen abundance that is expected near the end
of core-helium burning. The very high stellar temperature
that is needed to produce O vi emission is expected if WOs
are indeed the descendants of WC stars, as the envelope is
being stripped by the stellar wind and consecutively hotter layers are revealed. An alternative scenario is that WO
stars originate from higher mass progenitors. In this case
the stars are hotter and the O vi emission could purely be
an excitation effect, and a high oxygen abundance is not
necessarily implied (e.g., Hillier & Miller 1999).
1
SF2A 2015
S. Boissier, V. Buat, L. Cambr´
esy, F. Martins and P. Petit (eds)
THE PECULIAR ABUNDANCE PATTERN OF THE NEW HG-MN STAR HD 30085
R.Monier1, 2 , M.Gebran 3 , F.Royer 4 and R.E.M.Griffin 5
Abstract. Using high-dispersion, high-quality spectra of HD 30085 obtained with the echelle spectrograph
SOPHIE at l’Observatoire de Haute Provence, we show that this star contains strong lines of the s-process
elements Sr ii, Y ii and Zr ii. Line syntheses of the lines yield large overabundances of Sr, Y, Zr which are
characteristic of HgMn stars. The Sr-Y-Zr triad of abundances is inverted in HD 30085 compared to that
in our solar system. The violation of the odd-even rule suggests that physical processes such as radiative
diffusion, chemical fractionation and others must be at work in the atmosphere of HD 30085, and that the
atmosphere is stable enough to sustain them.
Keywords:
1
stars: individual, stars: Chemically Peculiar
Introduction
HD 30085, currently assigned a spectral type of A0 IV, is one of the 47 northern slowly-rotating early-A stars
stars studied by Royer et al. (2014). It shows strong lines of Mn ii and Hg ii, and recently Monier et al. (2015)
synthesized several lines of Mn ii, Fe ii and Hg ii which are present in spectra observed with SOPHIE, using
model atmospheres and spectrum synthesis that include hyperfine structure of various isotopes where relevant.
The synthetic spectra were adjusted iteratively to the observed high-resolution, high signal-to-noise spectra in
order to derive the abundances of those elements. The analysis yielded over-abundances of 40 times solar for
Mn and 32000 times solar for Hg, thus demonstrating unquestionably that the star needs to be re-classified as
an HgMn star. In this paper we focus on lines of Sr, Y, Zr which are also strong in the spectrum of HD 30085,
and derive the element abundances.
2
Observations and reduction
HD 30085 was observed twice at l’Observatoire de Haute Provence in February 2012 and December 2013, using
the high-resolution mode (R =75000) of SOPHIE. Three 15-minute exposures were obtained in February 2012
S
and coadded to create a mean spectrum with a N
ratio of about 316. A single 20-minute exposure was acquired
S
in December 2013, with a N of ∼300.
3
Lines of Sr ii, Zr ii and Y ii in HD 30085
The strongest lines of Sr ii, Y ii and Zr ii in our line catalogue are conspicuous in the SOPHIE spectra of
HD 30085. They are listed in Table 1 along with the measured equivalent width and derived abundance for
each transition. Only a few of these lines are unblended; most of the blends are with lines of Cr ii, Mn ii and
Fe ii, whose abundances were derived in Monier et al. (2015). Fig. 1 displays the resonance-line profile of Sr ii
at 4305 ˚
A and that of Zr ii at 4496 ˚
A to illustrate their strengths and the overabundances of those species.
1
2
3
4
5
LESIA, UMR 8109, Observatoire de Paris Meudon, Place J.Janssen, Meudon, France
Lagrange, UMR 7293, Universite de Nice Sophia, Nice, France
Department of Physics and Astronomy, Notre Dame University - Louaize, PO Box 72, Zouk Mikael, Lebanon
GEPI, UMR 8111, Observatoire de Paris Meudon, Place J.Janssen, Meudon, France
Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
c Soci´
et´
e Francaise d’Astronomie et d’Astrophysique (SF2A) 2015
c ESO 2015
Astronomy & Astrophysics manuscript no. 40erib
July 6, 2015
A novel and sensitive method for measuring very weak magnetic
fields of DA white dwarfs
A search for a magnetic field at the 250 G level in 40 Eri B
J. D. Landstreet1,2 , S. Bagnulo1 , G. G. Valyavin3 , D. Gadelshin3 , A. J. Martin1,4 , G. Galazutdinov5,6,3 , and E.
Semenko3,⋆
1
2
Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK e-mail: [email protected];[email protected]
Department of Physics & Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
3
Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchukskiy Region, 369167 Karachai-Cherkessian Republic, Russia
4
Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, UK
5
Instituto de Astronomia, Universidad Catolica del Norte, Av. Angamos 0610, Antofagasta Chile
6
Pulkovo Observatory, Pulkovskoe Shosse 65, Saint-Petersburg 196140, Russia
Received September 15, 1996; accepted March 16, 1997
ABSTRACT
Context. Searches for magnetic fields in white dwarfs have clarified both the frequency of occurrence and the global structure of the
fields found down to field strengths of the order of 500 kG. Below this level, the situation is still very unclear.
Aims. We are engaged in a project to find and study the weakest magnetic fields that are detectable in white dwarfs, in order to
empirically determine how the frequency of occurrence and the structure of fields present changes with field strength. In this paper
we report the successful testing of a very sensitive method of longitudinal field detection in DA white dwarfs. We use this method to
carry out an extremely sensitive search for magnetism in the bright white dwarf 40 Eri B.
Methods. The method of field measurement we use is to measure, at high spectral resolution, the polarisation signal V/I of the narrow
non-LTE line core in Hα in DA stars. This small feature provides a much higher amplitude polarisation signal than the broad Balmer
line wings. We test the usefulness of this technique by searching for a weak magnetic field in 40 Eri B.
Results. One hour of observation of I and V Stokes components of the white dwarf 40 Eri B using ESPaDOnS at the CFHT is found
to provide a standard error of measurement of the mean longitudinal magnetic field hBz i of about 85 G. This is the smallest standard
error of field measurement ever obtained for a white dwarf. The non-detections obtained are generally consistent with slightly less
accurate measurements of 40 Eri B obtained with ISIS at the WHT and the Main Stellar Spectrograph at SAO, in order to provide
comparison standards for the new method. These further measurements allow us to make a quantitative comparison of the relative
efficiencies of low-resolution spectropolarimetery (using most or all of the Balmer lines) with the new method (using only the core of
Hα).
Conclusions. The new method of field detection reaches the level of sensitivity that was expected. It appears that for suitable DA stars,
about the same field uncertainties can be reached with ESPaDOnS on the CFHT, in a given integration time, as with FORS on an 8-m
telescope, and uncertainties are a factor of two better than with low-resolution spectropolarimetry with other 4–6-m class telescopes.
However, even with this extraordinary sensitivity, there is no clear indication of the presence of any magnetic field in 40 Eri B above
the level of about 250 G.
Key words. techniques: polarimetric – techniques: spectroscopic – magnetic fields – stars: white dwarfs – stars: magnetic field
1. Introduction
Megagauss magnetic fields were first discovered in white
dwarfs in the 1970s by detection of broadband circular
(and soon afterwards linear) polarisation (Kemp et al. 1970;
Angel & Landstreet 1970a, 1971). It was already understood
that such huge fields, and indeed much weaker ones, could
⋆
Based in part on observations obtained at the Canada-FranceHawaii Telescope (CFHT) which is operated by the National Research
Council of Canada, the Institut National des Sciences de l’Univers of
the Centre National de la Recherche Scientifique of France, and the
University of Hawaii.
be detected by observing the Zeeman effect, for example
in Balmer lines (Angel & Landstreet 1970b), and since the
early 1970s the overwhelming majority of magnetic white
dwarfs have been discovered using this effect, which in practice is sensitive to fields from some kG to hundreds of MG
in white dwarfs (Landstreet 1992; Vanlandingham et al. 2005;
Kawka et al. 2007). At present many hundreds of magnetic
white dwarfs are known as a result of searches through the
thousands of white dwarf spectra obtained by the Sloan Digital
Sky Survey (Kepler et al. 2013). It appears that magnetic fields
1
ASTROPHYSICAL SOURCES OF STATISTICAL UNCERTAINTY IN PRECISION RADIAL VELOCITIES AND
THEIR APPROXIMATIONS
Thomas G. Beatty1,2 & B. Scott Gaudi3
ABSTRACT
We investigate various astrophysical contributions to the statistical uncertainty of precision radial
velocity measurements of stellar spectra. We first analytically determine the intrinsic uncertainty in
centroiding isolated spectral lines broadened by Gaussian, Lorentzian, Voigt, and rotational profiles,
finding that for all cases and assuming weak lines, the uncertainty is the line centroid is σV ≈
1/2
C Θ3/2 /(W I0 ), where Θ is the full-width at half-maximum of the line, W is the equivalent width,
and I0 is the continuum signal-to-noise ratio, with C a constant of order unity that depends on the
specific line profile. We use this result to motivate approximate analytic expressions to the total radial
velocity uncertainty for a stellar spectrum with a given photon noise, resolution, wavelength, effective
temperature, surface gravity, metallicity, macroturbulence, and stellar rotation. We use these relations
to determine the dominant contributions to the statistical uncertainties in precision radial velocity
measurements as a function of effective temperature and mass for main-sequence stars. For stars more
massive than ∼ 1.1 M⊙ we find that stellar rotation dominates the velocity uncertainties for moderate
and high resolution spectra (R & 30, 000). For less massive stars, a variety of sources contribute
depending on the spectral resolution and wavelength, with photon noise due to decreasing bolometric
luminosity generally becoming increasingly important for low-mass stars at fixed exposure time and
distance. In most cases, resolutions greater than 60,000 provide little benefit in terms of statistical
precision, although higher resolutions would likely allow for better control of systematic uncertainties.
We find that the spectra of cooler stars and stars with higher metallicity are intrinsically richer in
velocity information, as expected. We determine the optimal wavelength range for stars of various
spectral types, finding that the optimal region depends on the stellar effective temperature, but for
mid M-dwarfs and earlier the most efficient wavelength region is from 6000 ˚
A to 9000 ˚
A.
1. INTRODUCTION
Current generation radial velocity (RV) surveys for exoplanets mostly focus on relatively bright, V < 8.5, stars
using single-object spectrographs. Since they are using
single-object spectrographs, the surveys target one star
at a time and expose up to a desired continuum signalto-noise ratio (SNR). The V cut-off used by the RV surveys is generally driven by the faint limit of the Hipparcos results (e.g., Marcy et al. 2005); current RV surveys
deliberately pre-select all of their stars to ensure that
they will be dwarfs, chromospherically quiet, and do not
have known stellar companions – so that they will be
good RV targets. Initially the surveys largely restricted
their targets to single G- and K-dwarfs (e.g. the stellar sample described in Wright et al. 2004). This choice
was driven on the one side by the faintness of M-dwarfs,
which makes getting a high SNR in a reasonable exposure
time difficult, and on the other side by the rapid rotation of stars hotter than the Kraft Break (Kraft 1970) at
6250K, which widens the stellar lines and makes precision
RV measurements difficult. Over the last two decades,
RV surveys have thus surveyed effectively all of the single G- and K-dwarfs brighter than V < 8.5 for Jupitermass planets out to periods of 5.5 years (see, for example,
1 Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802
2 Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802
3 Department of Astronomy, The Ohio State University, 140
W. 18th Ave., Columbus, OH 43210
Cumming et al. 2008; Wright 2005).
In the last ten years, a new generation of RV surveys arose that targeted M-dwarfs (M2K and the Keck
M-dwarf Survey, Apps et al. 2010; Johnson et al. 2010),
fainter high-metallicity stars (N2K, Fischer et al. 2005),
and former A-stars that are now sub-giants (“Retired A
Stars,” Johnson et al. 2007) – to name a few examples.
These newer surveys use the same observing mode as previous surveys: single-object spectrographs and exposing
to a desired SNR. They are nearly complete for Jupitermass planets out to several hundred days, though they
have yet to survey all of the possible stars available.
As the field moves forward, next generation of RV surveys will target fainter stars in broader observing modes.
This will be enabled by the results from the GAIA mission, which will allow for vetting of target stars up to
V < 20 (de Bruijne 2012), and also by the exhaustion of
unsurveyed bright stars. There will also be a considerable demand for RV resources to follow-up planet candidates from the upcoming Transiting Exoplanet Survey
Satellite (TESS, Ricker et al. 2015) and Planetary Transits and Oscillations of stars (PLATO, Rauer et al. 2013)
missions, as well as residual planet candidates from the
Kepler (Borucki et al. 2010) mission. Importantly, these
photometric exoplanet surveys do not pre-select their
targets in the same manner as the traditional RV surveys (if they pre-select at all). Vetting the candidates
from these missions will therefore necessitate precision
RV measurements of stars that are potentially more active, hotter, more evolved, and rotating faster than the
Characterizing Rocky and Gaseous Exoplanets with 2-meter Class
Space-based Coronagraphs: General Considerations
arXiv:1507.00777v1 [astro-ph.EP] 2 Jul 2015
Tyler D. Robinson
NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
[email protected]
Karl R. Stapelfeldt
Exoplanets and Stellar Astrophysics Laboratory, Code 667, NASA Goddard Space Flight
Center, Greenbelt, MD 20771, USA
and
Mark S. Marley
NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
ABSTRACT
Several concepts now exist for small, space-based missions to directly characterize exoplanets in reflected light. While studies have been performed that
investigate the potential detection yields of such missions, little work has been
done to understand how instrumental and astrophysical parameters will affect
the ability of these missions to obtain spectra that are useful for characterizing
their planetary targets. Here, we develop an instrument noise model suitable
for studying the spectral characterization potential of a coronagraph-equipped,
space-based telescope. We adopt a baseline set of telescope and instrument parameters, including a 2 m diameter primary aperture, an operational wavelength
range of 0.4–1.0 µm, and an instrument spectral resolution of λ/∆λ = 70, and
apply our baseline model to a variety of spectral models of different planet types,
including Earth twins, Jupiter twins, and warm and cool Jupiters and Neptunes.
With our exoplanet spectral models, we explore wavelength-dependent planetstar flux ratios for main sequence stars of various effective temperatures, and
discuss how coronagraph inner and outer working angle constraints will influence the potential to study different types of planets. For planets most favorable
to spectroscopic characterization—cool Jupiters and Neptunes as well as nearby
Earth twins and super-Earths—we study the integration times required to achieve
Printed 6 July 2015
The orbital PDF: the dynamical state of Milky Way sized
haloes and the intrinsic uncertainty in the determination of
their masses
Jiaxin Han,1⋆ , Wenting Wang,1, Shaun Cole,1, Carlos S. Frenk,1
1 Institute
of Computational Cosmology, Department of Physics, University of Durham, South Road, Durham, DH1 3LE
6 July 2015
ABSTRACT
Using realistic cosmological simulations of Milky Way sized haloes, we study their
dynamical state and the accuracy of inferring their mass profiles with steady-state
models of dynamical tracers. We use a new method that describes the phase-space
distribution of a steady-state tracer population in a spherical potential without any
assumption regarding the distribution of their orbits. Applying the method to five
haloes from the Aquarius ΛCDM N-body simulation, we find that dark matter particles are an accurate tracer that enables the halo mass and concentration parameters
to be recovered with an accuracy of 5%. Assuming a potential profile of the NFW form
does not significantly affect the fits in most cases, except for halo A whose density
profile differs significantly from the NFW form, leading to a 30% bias in the dynamically fitted parameters. The existence of substructures in the dark matter tracers
only affects the fits by ∼ 1%. Applying the method to mock stellar haloes generated
by a particle-tagging technique, we find the stars are farther from equilibrium than
dark matter particles, yielding a systematic bias of ∼ 20% in the inferred mass and
concentration parameter. The level of systematic biases obtained from a conventional
distribution function fit to stars is comparable to ours, while similar fits to DM tracers
are significantly biased in contrast to our fits. In line with previous studies, the mass
bias is much reduced near the tracer half-mass radius.
Key words: dark matter – galaxies: haloes – galaxies: kinematics and dynamics –
Galaxy: fundamental parameters – methods: data analysis
1
INTRODUCTION
Dynamical modelling is of fundamental importance in the
determination of the mass distribution of dark matter
haloes. To constrain the total mass distribution or the gravitational potential, a large family of dynamical methods
work by fitting a proposed potential-dependent distribution
function (DF) to the observed phase-space distribution of
a tracer population. In such modelling, one should make as
few assumptions as possible so as to avoid biasing the results. In practice, a required minimal assumption is that the
system is in steady state, so that modelling the tracer DF
with a single observational snapshot is informative without
requiring the observation to take place at any special moment. However, most existing methods involve additional
assumptions, for example, about the distribution of orbits,
the functional form of the DF, or the spatial distribution
⋆
[email protected]
of tracer particles outside the observational window. In a
previous paper (Han et al. 2015, hereafter Paper I), we developed a method that can be used to infer the potential
while only making the assumption that the tracer population is in a steady state. In particular, taking a spherical potential as an example, we have shown that the steady-state
property translates into a fundamental orbital Probability
Density Function (oPDF), which provides enough information to enable the inference of the halo potential. Applying
this method to a set of steady-state tracers in an NFW potential generated from Monte-Carlo simulations, we showed
that the method is able to recover the true potential. While
spherical symmetry is assumed, all the steps of the method
can be generalized to non-spherical cases.
A realistic halo from cosmological simulations or one in
the real universe may violate the assumptions of our method
in several ways. For example, spherical symmetry is only
approximate since we know haloes are triaxial (Frenk et al.
1988; Jing & Suto 2002). Also, the potential and the distri-
Printed 6 July 2015
The orbital PDF: general inference of the gravitational
potential from steady-state tracers
Jiaxin Han,1⋆ Wenting Wang,1 Shaun Cole,1 Carlos S. Frenk1
1 Institute
of Computational Cosmology, Department of Physics, University of Durham, South Road, Durham, DH1 3LE
6 July 2015
ABSTRACT
We develop two general methods to infer the gravitational potential of a system using
steady-state tracers, i.e., tracers with a time-independent phase-space distribution.
Combined with the phase-space continuity equation, the time independence implies a
universal Orbital Probability Density Function (oPDF) dP (λ|orbit) ∝ dt, where λ is
the coordinate of the particle along the orbit. The oPDF is equivalent to Jeans theorem, and is the key physical ingredient behind most dynamical modelling of steadystate tracers. In the case of a spherical potential, we develop a likelihood estimator that
fits analytical potentials to the system, and a non-parametric method (“Phase-Mark”)
that reconstructs the potential profile, both assuming only the oPDF. The methods
involve no extra assumptions about the tracer distribution function and can be applied
to tracers with any arbitrary distribution of orbits, with possible extension to nonspherical potentials. The methods are tested on Monte-Carlo samples of steady-state
tracers in dark matter haloes to show that they are unbiased as well as efficient. A
fully documented C/Python code implementing our method is freely available at a
GitHub repository linked from http://icc.dur.ac.uk/data/#oPDF.
Key words: dark matter – galaxies: haloes – galaxies: kinematics and dynamics –
Galaxy: fundamental parameters – methods: data analysis
1
INTRODUCTION
Since dark matter does not emit or absorb electromagnetic
radiation, gravitational modelling is of fundamental importance to the determination of the dark matter distribution.
Such modelling can be performed using either gravitational
lensing (e.g., Bartelmann 2010; Han et al. 2014), or the dynamics of tracers (e.g., stars or galaxies; see Courteau et al.
2014 for a recent review on galaxy mass inferences).
One straightforward way to perform dynamical modelling is to fit a proposed phase-space distribution function
(DF) to the observed positions and velocities of tracer particles. Thanks to Jeans theorem (see e.g. Binney & Tremaine
2008), which states that functions of the integrals of motion, J, are solutions of the Boltzmann equation, one can
simply consider distribution functions of the form f (J). Under certain conditions,
one can invert the observed density
R
profile, ρ = f (J)d3 v, of the tracer to construct a specific
family of f (J) (e.g., Eddington 1916; Osipkov 1979; Merritt
1985; Cuddeford 1991; hereafter referred to as density profile inversion). The density profile inversion depends on the
potential of the halo, and the resulting f is thus poten⋆
[email protected]
tial dependent. However, some further assumptions about
the functional form of f (J) are required to perform the inversion. These assumptions are typically motivated either
empirically (e.g., Wojtak et al. 2008), or by mathematical
simplicity (e.g., Evans & An 2006).
For example, in Wilkinson & Evans (1999) and
Wang et al. (2015), solutions of the form f (E, L) =
f (E)L−2β were used to constrain the potential of the
Galactic halo, where E and L are the energy and angular momentum of tracer particles. In particular, Wang et al.
(2015) applied the f (E, L) method to mock stellar
haloes (Cooper et al. 2010) constructed from the Aquarius
simulations of ΛCDM galactic haloes (Springel et al. 2008)
and found significant biases in the fitted masses. These biases suggest that the proposed f (E, L) DF does not describe
well the observed phase-space distribution of the mock stars.
However, it is not clear whether the discrepancy is due to
departures from dynamical equilibrium by the stars within
the halo potential, or to the lack of generality in the proposed f (E, L) functional form used to describe the distribution (e.g., Binney & Mamon 1982). In the former case, the
stars would represent an intrinsically biased tracer, and the
modelling of their distribution would not give the correct
potential. In the latter case, one can still hope to find a DF
1
arXiv:1507.00761v1 [astro-ph.EP] 2 Jul 2015
The Composition of Comets
Anita L. Cochran1 , Anny-Chantal Levasseur-Regourd2 ,Martin Cordiner3 ,
Edith Hadamcik4 , Jérémie Lasue5 , Adeline Gicquel3,9 ,
David G. Schleicher6 , Steven B. Charnley3 , Michael J. Mumma3 ,
Lucas Paganini3 , Dominique Bockelée-Morvan7 , Nicolas Biver7 , Yi-Jehng Kuan8
1. The University of Texas, McDonald Observatory, Austin, TX USA
2. UPMC, LATMOS, Paris, France
3. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
4. UPMC, LATMOS, Guyancourt, France
5. UPS, IRAP, Toulouse, France
6. Lowell Observatory, Flagstaff, AZ, USA
7. LESIA, Observatoire de Paris, Meudon, France
8. National Taiwan Normal University, Taiwan, ROC
9. MPS, G¨
ottingen, Germany
2
Abstract
This paper is the result of the International Cometary Workshop, held in Toulouse,
France in April 2014, where the participants came together to assess our knowledge of
comets prior to the ESA Rosetta Mission. In this paper, we look at the composition of
the gas and dust from the comae of comets. With the gas, we cover the various taxonomic
studies that have broken comets into groups and compare what is seen at all wavelengths.
We also discuss what has been learned from mass spectrometers during flybys. A few
caveats for our interpretation are discussed. With dust, much of our information comes
from flybys. They include in situ analyses as well as samples returned to Earth for
laboratory measurements. Remote sensing IR observations and polarimetry are also
discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and
Philae lander will bring to bear to improve our understanding of comet 67P/ChuryumovGerasimenko as “ground-truth” for our previous comprehensive studies. Finally, we
summarize some of the inital Rosetta Mission findings.
1
Introduction
Comets are leftovers from when our Solar System formed. These icy bodies were formed
out of the solar nebula in the outer regions of our Solar System. Many of these bodies
were swept up and incorporated into the giant planets. The remnants of planetary formation were either ejected from the nascent Solar System, were gravitationally perturbed
to the Oort cloud or remained in reservoirs past the orbit of Neptune. The comets we see
today have undergone little change from their primordial states. Comets, therefore, represent important objects to study in order to determine constraints suitable for models
of the early solar nebula.
Whipple (1950) first described comets as “dirty snowballs” that are composed of
a mixture of ices and dust. The mass is approximately equally divided into ices and
dust. As the comets approach the Sun, they are heated and the ices sublime. Since
comets are small (generally a few to 10s of km), the resultant gas is not bound to the
nucleus and expands outwards into the vacuum of space. When it leaves the nucleus, the
gas carries with it some of the solid particles (refractory grains, refractory organics, icy
grains, agregates of ice and dust). The resultant material forms the cometary comae (and
sometimes tails) that are so prominent when a comet is in the inner Solar System. We do
not observe the nucleus composition directly, except when observing from a spacecraft
flying past or during a rendezvous mission. What we generally observe is the gas in the
coma and the light reflected off the refractories and we infer composition from those.
The gases that come directly from the nucleus first flow through a region near the
nucleus where the gas densities are sufficiently high that collisions, and thus chemical
Printed 6 July 2015
Study of X-ray emission from the old open cluster, M67
K.
P. Mooley 1 and K. P. Singh2
1
2
California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125
Tata Institute of Fundamental Research, Mumbai 400 005, India
6 July 2015
ABSTRACT
We present an X-ray analysis of a 4 Gyr old open cluster, M67, using archival XMM-Newton
data. The aim of this study was to find new X-ray members of M67, and to use the updated
member list for studying X-ray variability, and derive the X-ray luminosity functions (XLFs)
of different stellar types and compare them with other star clusters of similar age. We report
the detection of X-ray emission from 25 members of M67, with membership based primarily
on their proper motion, of which one X-ray source is a new member. Supplementing this study
with previous ROSAT and Chandra studies of M67, and using the most recent proper motion
study by Vereshchagin et al., we have compiled a revised list of X-ray emitting members
of M67 consisting of 43 stars. Sixteen of these are known RS CVn type binaries having
orbital periods < 10 days, and near-circular orbits, 5 are contact binaries with orbital periods
< 6 hours, 5 are yellow and blue stragglers, 2 are Algol-type binaries, and one source is a
cataclysmic variable. Fourteen members do not have any orbital information and cannot be
classified. Fourteen of the X-ray sources detected do not have any optical counterpart down
to a magnitude of V ' 22, and their membership is uncertain. Finally, we report the X-ray
luminosity functions of RS CVn type and other types of stars in M67 and compare them with
other open clusters of intermediate-to-old age.
Key words: stars : activity – binaries : general – stars : blue stragglers – open clusters and
associations: individual: M67 – X-rays : binaries
1
INTRODUCTION
Open clusters are useful for studying coeval and comoving populations of stars within the Galactic disk. X-ray studies of stars in open
clusters offer an insight into their coronal activity and/or accretion
phenomena. As clusters age, the spin down of stars causes X-ray
emission to diminish in general, and thus, in the X-rays, revealing
active coronae primarily from stars spun-up in binary systems (the
age-rotation-activity correlation; e.g. Pallavicini 1989; Randich
1997; Gudel et al. 2004) or from systems undergoing accretion. Accordingly, X-rays from old (a few Gyr or older) open clusters are
unique probes of magnetically active (RS CVn, BY Dra, W UMa,
FK Com, Algol) and mass-transfer (CVs, L/HMXBs) binary systems within the Galactic disk (Belloni et al. 1998; Verbunt 1999;
van den Berg 2013).
Past studies of old open clusters, NGC 6791 (∼8 Gyr; van den
Berg et al. 2013), NGC 188 (∼6 Gyr; Belloni et al. 1998), and M67
(Belloni et al. 1993, 1998; van den Berg et al. 2004), reveal that RS
CVns, CVs, and sub-giant stars dominate the X-ray emission, while
peculiar objects such as blue and yellow stragglers are rare and thus
do not contribute much.
M67 is a 4.2±0.6 Gyr-old open cluster at 850±30 pc having
a small reddening value (EB−V ≈ 0.04) (Sarajedini et al. 1999;
Yadav et al. 2008). On account of the extensive optical data available for M67, this cluster is well suited for study in X-rays. Proper© 0000 RAS
motion studies of M67 to establish cluster membership have been
carried out by several groups (Sanders 1977; Girard et al. 1989;
Zhao et al. 1993; Yadav et al. 2008; Vereshchagin et al. 2014). In
this paper we use cluster membership information from Vereshchagin et al. (2014), a revised version of the Yadav et al. (2008) catalog
reaching down to V∼22 mag, containing 659 members. Two published X-rays studies of M67 exist. Belloni et al. (1993, 1998) presented ROSAT PSPC observations covering ∼0.5 deg2 of the M67
field, while van den Berg et al. (2004) analyzed Chandra ACIS observations covering ∼0.1 deg2 but with a limiting flux of about 40
times lower than the ROSAT observations.
Here, we present the results of two XMM-Newton observations of M67 having fields of view and limiting flux intermediate
to that afforded by the ROSAT and Chandra observations. The paper is organised as follows. In Section 2, we discuss the X-ray data
and processing. The optical, ROSAT and Chandra counterparts of
our X-ray sources are given in Section 3. The spectral hardness and
variability analysis is in Section 4. Section 5 gives notes on individual classes of X-ray sources, and we conclude with a discussion
in Section 6.
Accepted for Publication in the Astronomical Journal: June 22, 2015
A SPECTROSCOPIC ANALYSIS OF THE GALACTIC GLOBULAR CLUSTER NGC 6273 (M19)∗
Christian I. Johnson1,2 , R. Michael Rich3 , Catherine A. Pilachowski4 , Nelson Caldwell1 , Mario Mateo5 , John
I. Bailey, III5 , and Jeffrey D. Crane6
Accepted for Publication in the Astronomical Journal: June 22, 2015
ABSTRACT
A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These “anomalous” clusters, which we refer to as “iron–complex” clusters, are
differentiated from normal clusters by exhibiting large (&0.10 dex) intrinsic metallicity dispersions,
complex sub–giant branches, and correlated [Fe/H] and s–process enhancements. In order to further
investigate this phenomenon, we have measured radial velocities and chemical abundances for red
giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities
and abundances were determined using high resolution (R∼27,000) spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan–Clay 6.5m telescope
at Las Campanas Observatory. We find that NGC 6273 has an average heliocentric radial velocity of
+144.49 km s−1 (σ=9.64 km s−1 ) and an extended metallicity distribution ([Fe/H]=–1.80 to –1.30)
composed of at least two distinct stellar populations. Although the two dominant populations have
similar [Na/Fe], [Al/Fe], and [α/Fe] abundance patterns, the more metal–rich stars exhibit significant
[La/Fe] enhancements. The [La/Eu] data indicate that the increase in [La/Fe] is due to almost pure
s–process enrichment. A third more metal–rich population with low [X/Fe] ratios may also be present.
Therefore, NGC 6273 joins clusters such as ω Centauri, M 2, M 22, and NGC 5286 as a new class of
iron–complex clusters exhibiting complicated star formation histories.
Subject headings: stars: abundances, globular clusters: general, globular clusters: individual (NGC
6273, M 19)
1. INTRODUCTION
Galactic globular clusters are proving to be a rich and
diverse population. These objects are generally older
than about 10 Gyr (σ∼3 Gyr; e.g., Rosenberg et al. 1999;
Salaris & Weiss 2002; De Angeli et al. 2005; Mar´ın–
Franch et al. 2009; VandenBergh et al. 2013), but also
span about a factor of 300 in metallicity (e.g., Zinn &
West 1984; Harris 1996; Carretta & Gratton 1997; Kraft
& Ivans 2003; Carretta et al. 2009a). Early high resolution spectroscopic work revealed that clusters typically
contain stars with similar heavy element abundances,
most notably [Fe/H]7 , but with >0.5–1.0 dex star–to–
star abundance variations for elements lighter than about
Si (e.g., Cohen 1978; Peterson 1980; Sneden et al. 1991;
Kraft et al. 1992; Pilachowski et al. 1996; Gratton
et al. 2001). Furthermore, it was discovered that the
light element abundance variations are (anti–)correlated
∗ THIS
PAPER INCLUDES DATA GATHERED WITH THE 6.5
METER MAGELLAN TELESCOPES LOCATED AT LAS CAMPANAS OBSERVATORY, CHILE.
1 Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, MS–15, Cambridge, MA 02138, USA; [email protected]; [email protected]
2 Clay Fellow
3 Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA;
[email protected]
4 Astronomy Department, Indiana University Bloomington,
Swain West 319, 727 East 3rd Street, Bloomington, IN 47405–
7105, USA; [email protected]
5 Department of Astronomy, University of Michigan, Ann Arbor,
MI 48109, USA; [email protected]; [email protected]
6 The Observatories of the Carnegie Institution for Science,
Pasadena, CA 91101, USA; [email protected]
7
[A/B]≡log(NA /NB )star –log(NA /NB )⊙
and
log
ǫ(A)≡log(NA /NH )+12.0 for elements A and B.
with one another, and that the correlation patterns,
such as the O–Na anti–correlation and Na–Al correlation,
are evidence that the gas from which the present–day
low mass globular cluster stars formed was subjected to
high–temperature proton–capture nucleosynthesis (e.g.,
Denisenkov & Denisenkova 1990; Langer et al. 1993;
Prantzos et al. 2007). Except for a few notable cases
(Koch et al. 2009; Caloi & D’Antona 2011; Villanova
et al. 2013), recent large sample spectroscopic surveys
(e.g., Carretta et al. 2009b,c) have built upon this early
work and cemented the idea that the correlated light element abundance variations are a characteristic common
to perhaps all Galactic globular clusters (see also reviews
by Kraft 1994; Gratton et al. 2004; 2012). These spectroscopic observations provided the first evidence that
globular clusters may not be simple stellar populations.
Concurrent with spectroscopic work has been the revelation that, when observed using appropriate filter combinations, the color–magnitude diagrams of many globular clusters exhibit multiple, often discreet, photometric
sequences that can extend from the main–sequence to the
asymptotic giant branch (AGB; e.g., Piotto et al. 2007,
2012, 2015; Lardo et al. 2011; Milone et al. 2012a,b,
2015; Monelli et al. 2013; Cummings et al. 2014; Lim
et al. 2015). Since many of the filters used are sensitive to a star’s light element composition (e.g., Bond &
Neff 1969), the combined spectroscopic evidence of large
star–to–star light element abundance variations and photometric evidence of multiple color–magnitude diagram
sequences reveals that nearly all clusters host more than
one stellar population. For clusters exhibiting a negligible spread in [Fe/H], the various populations are often
categorized by their light element chemistry as “primor-
Printed July 6, 2015
Machine Learning based photometric redshifts for the KiDS ESO
DR2 galaxies
S. Cavuoti1⋆ , M. Brescia1, C. Tortora1 , G. Longo2, N. R. Napolitano1 , M. Radovich3,
F.
La Barbera1, M. Capaccioli4, J. T.A. de Jong5, F. Getman1 , A. Grado1 , M. Paolillo2 .
1
Astronomical Observatory of Capodimonte - INAF, via Moiariello 16, I-80131, Napoli, Italy
of Physics, University Federico II, via Cinthia 6, 80126 Napoli, Italy
3 Astronomical Observatory of Padua, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
4 VSTCen - INAF, via Moiariello 16, I-80131, Napoli, Italy
5 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
2 Department
Accepted ; Received ; in original form
ABSTRACT
We estimated photometric redshifts (zphot ) for more than 1.1 million galaxies of the ESO
Public Kilo-Degree Survey (KiDS) Data Release 2. KiDS is an optical wide-field imaging
survey carried out with the VLT Survey Telescope (VST) and the OmegaCAM camera, which
aims at tackling open questions in cosmology and galaxy evolution, such as the origin of
dark energy and the channel of galaxy mass growth. We present a catalogue of photometric redshifts obtained using the Multi Layer Perceptron with Quasi Newton Algorithm
(MLPQNA) model, provided within the framework of the DAta Mining and Exploration
Web Application REsource (DAMEWARE). These photometric redshifts are based on a
spectroscopic knowledge base which was obtained by merging spectroscopic datasets from
GAMA (Galaxy And Mass Assembly) data release 2 and SDSS-III data release 9. The overall
1σ uncertainty on ∆z = (z spec − z phot )/(1 + z spec ) is ∼ 0.03, with a very small average bias of
∼ 0.001, a NMAD of ∼ 0.02 and a fraction of catastrophic outliers (|∆z| > 0.15) of ∼ 0.4%.
Key words: techniques: photometric - galaxies: distances and redshifts - galaxies: photometry
1 INTRODUCTION
Photometric redshifts (zphot ) derived from multi-band digital
surveys are crucial to a variety of cosmological applications (Scranton et al. 2005; Myers et al. 2006; Hennawi et al. 2006;
Giannantonio et al. 2008). In the last years a plethora of methods
has been developed to estimate zphot (cf. Hildebrandt et al. 2010),
but the advent of a new generation of photometric surveys (to
quote just a few, Pan-STARRS: Kaiser 2004; Euclid: Laureijs et al.
2011; KiDS1 : de Jong et al. 2013) demands for higher accuracy
(Brescia et al. 2014b).
The evaluation of photometric redshifts requires the mapping of the photometric space into the spectroscopic redshift space.
All methods, one way or the other, require the use of a Knowledge Base (KB) consisting in a set of templates, and differ mainly
in the following aspects: (i) the way in which the KB is constructed
(spectroscopic redshifts or, rather, empirically or theoretically derived spectral energy distributions or SEDs), and (ii) the adopted
⋆
1
http://kids.strw.leidenuniv.nl/
interpolation/fitting algorithm. Methods based on the interpolation
of a spectroscopic KB are usually labeled as empirical.
Many different implementations of empirical methods exist and we shall recall just a few: i) polynomial fitting
(Connolly et al. 1995); ii) nearest neighbors (Csabai et al. 2003);
iii) neural networks (D’Abrusco et al. 2007; Yéche et al. 2010 and
references therein); iv) support vector machines (Wadadekar 2005);
v) regression trees (Carliles et al. 2010); vi) gaussian processes
(Way & Srivastava 2006; Bonfield et al. 2010), and vii) diffusion
maps (Freeman et al. 2009).
In this paper we discuss the derivation of photometric redshifts for the galaxies in the Kilo-Degree Survey (KiDS) data release 2 (de Jong et al. 2015). KiDS is an ESO public survey, based
on the VLT Survey Telescope (Capaccioli & Schipani 2011) with
the OmegaCAM camera (Kuijken 2011), that will image 1500
square degrees in four filters (u, g, r, i), in single epochs per filter. The high spatial resolution of VST (0.2′′ /pixel), the photometric depth and area covered make it a front-edge tool for weak
gravitational lensing and galaxy evolution studies. The measurement of unbiased and high-quality zphot is a crucial step to pursue
many of the scientific goals which motivated the KiDS survey
(de Jong et al. 2015).
Searching for galaxy clusters in the VST-KiDS Survey
M. Radovich1, E. Puddu2, F. Bellagamba3, L. Moscardini3, M. Roncarelli3 , F. Getman2, A. Grado2
and the KiDS collaboration
1
INAF - OAPD
2 INAF - OACN
3 Dept. of Physics and Astronomy (DIFA), University of Bologna
Abstract We present the methods and first results
of the search for galaxy clusters in the Kilo Degree Survey (KiDS). The adopted algorithm and
the criterium for selecting the member galaxies
are illustrated. Here we report the preliminary results obtained over a small area (7 sq. degrees),
and the comparison of our cluster candidates with
those found in the RedMapper and SZ Planck catalogues; the analysis to a larger area (148 sq. degrees) is currently in progress. By the KiDS cluster
search, we expect to increase the completeness of
the clusters catalogue to z = 0.6 − 0.7 compared to
RedMapper.
complete catalogue of galaxy clusters spanning
a broad range of redshifts would be important
to understand the history of cosmic structure
formation, and to provide accurate measurements of cosmological parameters, in particular
σ8 and ΩM . In this perspective, it becomes
crucial the role of extragalactic surveys that
can supply a statistically significant sample for
the detection of clusters. Among the already
available surveys of different sizes and depths,
one of the landmarks is the Sloan Digital Sky
Survey (SDSS; [15]), which allows to probe the
low-redshift Universe over an area of 20, 000
sq. degrees up to z ∼ 0.45. The Kilo Degree
Survey (KiDS; http://kids.strw.leidenuniv.nl/)
and the Dark Energy Survey (DES;
1 Introduction
http://www.darkenergysurvey.org/) will allow
to obtain samples of clusters spanning a wider
range of redshift and mass, thanks to their superior
Clusters of galaxies are described as the most depth and image quality. When completed, KiDS
massive gravitationally bound quasi-equilibrium will cover 1500 sq. degrees in the ugri bands, with
structures in the Universe. They represent a optimal seeing conditions in the r−band; DES
powerful tool for cosmology ([1]): a large and
1
Astronomy & Astrophysics manuscript no. kids-dr1dr2-arxiv
July 6, 2015
c
ESO
2015
The first and second data releases of the Kilo-Degree Survey
Jelte T. A. de Jong1 , Gijs A. Verdoes Kleijn2 , Danny R. Boxhoorn2 , Hugo Buddelmeijer2 , Massimo Capaccioli3 , Fedor
Getman3 , Aniello Grado3 , Ewout Helmich1 , Zhuoyi Huang3 , Nancy Irisarri1 , Konrad Kuijken1 , Francesco LaBarbera3 ,
John P. McFarland2 , Nicola R. Napolitano3 , Mario Radovich4 , Gert Sikkema2 , Edwin A. Valentijn2 , Kor G. Begeman2 ,
Massimo Brescia3 , Stefano Cavuoti3 , Ami Choi5 , Oliver-Mark Cordes6 , Giovanni Covone7 , Massimo Dall’Ora3 ,
Hendrik Hildebrandt6 , Giuseppe Longo7 , Reiko Nakajima6 , Maurizio Paolillo7, 8 , Emanuella Puddu3 , Agatino Rifatto3 ,
Crescenzo Tortora3 , Edo van Uitert6, 9 , Axel Buddendiek6 , Joachim Harnois-Déraps10 , Thomas Erben6 , Martin B.
Eriksen1 , Catherine Heymans5 , Henk Hoekstra1 , Benjamin Joachimi9 , Thomas D. Kitching11 , Dominik Klaes6 , Léon
V. E. Koopmans2 , Fabian Köhlinger1 , Nivya Roy7 , Cristóbal Sifón1 , Peter Schneider6 , Will J. Sutherland12 , Massimo
Viola1 , and Willem-Jan Vriend2
1
2
3
4
5
6
7
8
9
10
11
12
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, the Netherlands
e-mail: [email protected]
Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands
INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello 16 -80131 Napoli, Italy
INAF - Osservatorio Astronomico di Padova, via dell’Osservatorio 5, 35122 Padova, Italy
Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill,
Edinburgh EH9 3HJ, United Kingdom
Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany
Department of Physics, University Federico II, Via Cinthia 6, 80126 Napoli, Italy
Agenzia Spaziale Italiana - Science Data Center, Via del Politecnico snc, 00133 Roma, Italy
Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
Department of Physics and Astronomy, University of British Columbia, BC V6T 1Z1, Canada
Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, United Kingdom
School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom
ABSTRACT
Context. The Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried out with the VLT Survey Telescope and
the OmegaCAM camera. KiDS will image 1500 square degrees in four filters (ugri), and together with its near-infrared counterpart
VIKING will produce deep photometry in nine bands. Designed for weak lensing shape and photometric redshift measurements, the
core science driver of the survey is mapping the large-scale matter distribution in the Universe back to a redshift of ∼0.5. Secondary
science cases are manifold, covering topics such as galaxy evolution, Milky Way structure, and the detection of high-redshift clusters
and quasars.
Aims. KiDS is an ESO Public Survey and dedicated to serving the astronomical community with high-quality data products derived
from the survey data, as well as calibration data for use by others. Dissemination of data products is crucial for enabling independent
confirmation of the scientific value of the survey data. To these ends, public data releases will be made on a yearly basis, the first two
of which are presented here. The achieved data quality and initial scientific utilization are reviewed in order to validate the survey
data.
Methods. A dedicated pipeline and data management system based on the Astro-WISE software system, combined with newly
developed masking and source classification software, is used for the production of the data products described here. Early science
projects based on the data products in the first two data releases and preliminary results are outlined.
Results. For a total of 148 survey tiles (≈160 sq.deg.) astrometrically and photometrically calibrated, coadded ugri images have been
released, accompanied by weight maps, masks, source lists, and a multi-band source catalogue. Early scientific results include the
detection of nine high-z QSOs, fifteen candidate strong gravitational lenses, high-quality photometric redshifts and galaxy structural
parameters for hundreds of thousands of galaxies.
Key words. methods: observational – astronomical data bases: surveys – galaxies: general – cosmology: large-scale structure of
Universe
1. Introduction
Sensitive, wide-field astronomical surveys have proven to be extremely useful scientific resources, not only for the specific scientific use cases they are designed for, but also due to their legacy
value and as source-finders for the largest telescopes. With the
arrival of VISTA in 2010 and the VLT Survey Telescope (VST)
in 2011, the European astronomical community now has access
to dedicated survey telescopes both in the infrared and the optical. During the first years of operations the majority of observing time on both telescopes is dedicated to a number of public surveys, large observational programs selected by ESO that
serve the astronomical community with regular public data re-
Article number, page 1 of 26
Printed 6 July 2015
Evolution and nucleosynthesis of helium-rich asymptotic
giant branch models
Luke J. Shingles1? , Carolyn L. Doherty2 , Amanda I. Karakas1 , Richard J. Stancliffe3 ,
John
C. Lattanzio2 , and Maria Lugaro4
1
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Centre for Astrophysics (MoCA), School of Physics and Astronomy, Monash University, Victoria 3800, Australia
3 Argelander-Institut f¨
ur Astronomie, University of Bonn, Auf dem H¨
ugel 71, 53121 Bonn, Germany
4 Konkoly Observatory, Hungarian Academy of Sciences, PO Box 67, H-1525 Budapest, Hungary
2 Monash
Accepted 2015 July 2. Received 2015 July 2; in original form 2015 April 29
ABSTRACT
There is now strong evidence that some stars have been born with He mass fractions
as high as Y ≈ 0.40 (e.g., in ω Centauri). However, the advanced evolution, chemical
yields, and final fates of He-rich stars are largely unexplored. We investigate the
consequences of He-enhancement on the evolution and nucleosynthesis of intermediatemass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M with a metallicity
of Z = 0.0006 ([Fe/H] ≈ −1.4). We compare models with He-enhanced compositions
(Y = 0.30, 0.35, 0.40) to those with primordial He (Y = 0.24). We find that the
minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores
decreases from above our highest mass of 6 M to ∼ 4–5 M with Y = 0.40. We
also model the production of trans-Fe elements via the slow neutron-capture process
(s-process). He-enhancement substantially reduces the third dredge-up efficiency and
the stellar yields of s-process elements (e.g., 90% less Ba for 6 M , Y = 0.40). An
exception occurs for 3 M , where the near-doubling in the number of thermal pulses
with Y = 0.40 leads to ∼ 50% higher yields of Ba-peak elements and Pb if the 13 C
neutron source is included. However, the thinner intershell and increased temperatures
at the base of the convective envelope with Y = 0.40 probably inhibit the 13 C neutron
source at this mass. Future chemical evolution models with our yields might explain
the evolution of s-process elements among He-rich stars in ω Centauri.
Key words: stars: AGB and post-AGB stars — stars: evolution — nuclear reactions,
nucleosynthesis, abundances
1
INTRODUCTION
Stars that evolve through the asymptotic giant branch (AGB)
play a crucial role in the chemical evolution of stellar populations and galaxies. These stars experience a complex sequence
of He-shell instabilities, mixing, and mass loss processes that
combine to eject material that has undergone H and He
burning, and enrichment in heavy elements produced by the
slow neutron-capture process (s-process). Understanding the
chemical contribution by stars to the interstellar medium is
a prerequisite for building models of chemical evolution (e.g.,
Chiappini et al. 2001; Kobayashi et al. 2011).
The most important parameter governing stellar evolution is the initial mass, with a secondary role played by
chemical composition. Chemical composition typically refers
?
to the mass fraction of metals (Z), but the mass fraction
of He (Y ) also has major consequences for stellar evolution.
However, the stellar evolution and nucleosynthesis that take
place during the AGB phase are relatively unexplored for
He-rich initial compositions. For a detailed introduction to
AGB evolution and nucleosynthesis with normal He content,
we refer the reader to the reviews by Herwig (2005) and
Karakas & Lattanzio (2014).
There is strong evidence that some stars have been born
with substantial He enrichments above the primordial He
mass fraction of Y ≈ 0.24 predicted from big bang nucleosynthesis. The most direct method to measure He abundances is
to use spectroscopy, however the application to He is severely
hindered by the lack of He lines in cool stars, and the effects
of gravitational settling in hot stars. Still, direct detections of
He have been made, such as those by Pasquini et al. (2011),
who used the He I 10830 ˚
A line in two stars with different
MNRAS 000, 1–35 (2015)
Preprint 6 July 2015
Compiled using MNRAS LATEX style file v3.0
Gravitational Lensing Analysis of the Kilo Degree Survey
Konrad Kuijken1? , Catherine Heymans2 , Hendrik Hildebrandt3 , Reiko Nakajima3 ,
Thomas Erben3 , Jelte T.A. de Jong1 , Massimo Viola1 , Ami Choi2 , Henk Hoekstra1 ,
Lance Miller4 , Edo van Uitert3,5 , Alexandra Amon2 , Chris Blake6 , Margot Brouwer1 ,
Axel Buddendiek3 , Ian Fenech Conti7,8 , Martin Eriksen1 , Aniello Grado9 ,
Joachim Harnois-Déraps10 , Ewout Helmich1 , Ricardo Herbonnet1 , Nancy Irisarri1 ,
Thomas Kitching11 , Dominik Klaes3 , Francesco Labarbera9 , Nicola Napolitano9 ,
Mario Radovich12 , Peter Schneider3 , Cristóbal Sifón1 , Gert Sikkema13 , Patrick Simon3 ,
Alexandru
Tudorica3 , Edwin Valentijn13 , Gijs Verdoes Kleijn13 , Ludovic van Waerbeke10 .
1
Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.
Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ,
3 Argelander Institute for Astronomy, University of Bonn, Auf dem H¨
ugel 71, 53121 Bonn, Germany.
4 Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH, UK.
5 University College London, Gower Street, London WC1E 6BT, UK.
6 Centre for Astrophysics & Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia.
7 Institute of Space Sciences and Astronomy (ISSA), University of Malta, Msida MSD 2080.
8 Department of Physics, University of Malta, Msida, MSD 2080, Malta.
9 INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy.
10 Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, V6T 1Z1, BC, Canada.
11 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK.
12 INAF - Osservatorio Astronomico di Padova, via dell’Osservatorio 5, 35122 Padova, Italy.
13 Kapteyn Institute, University of Groningen, PO Box 800, NL 9700 AV Groningen.
2 Scottish
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
The Kilo-Degree Survey (KiDS) is a multi-band imaging survey designed for cosmological studies from weak lensing and photometric redshifts. It uses the ESO VLT
Survey Telescope with its wide-field camera OmegaCAM. KiDS images are taken in
four filters similar to the SDSS ugri bands. The best-seeing time is reserved for deep
r-band observations that reach a median 5-σ limiting AB magnitude of 24.9 with a
median seeing that is better than 0.700 .
Initial KiDS observations have concentrated on the GAMA regions near the celestial
equator, where extensive, highly complete redshift catalogues are available. A total of
101 survey tiles, one square degree each, form the basis of the first set of lensing analyses, which focus on measurements of halo properties of GAMA galaxies. 9 galaxies per
square arcminute enter the lensing analysis, for an effective inverse shear variance of
69 per square arcminute. Accounting for the shape measurement weight, the median
redshift of the sources is 0.53.
KiDS data processing follows two parallel tracks, one optimized for galaxy shape measurement (for weak lensing), and one for accurate matched-aperture photometry in
four bands (for photometric redshifts). This technical paper describes how the lensing and photometric redshift catalogues have been produced (including an extensive
description of the Gaussian Aperture and Photometry pipeline), summarizes the data
quality, and presents extensive tests for systematic errors that might affect the lensing
analyses. We also provide first demonstrations of the suitability of the data for cosmological measurements, and explain how the shear catalogues were blinded to prevent
confirmation bias in the scientific analyses.
The KiDS shear and photometric redshift catalogues, presented in this paper, are
released to the community through http://kids.strw.leidenuniv.nl.
Key words: cosmology: observations – gravitational lensing: weak – galaxies: photometry – surveys
c 2015 The Authors
?
MNRAS 000, 1–?? (2015)
The masses of satellites in GAMA galaxy groups from 100 square
degrees of KiDS weak lensing data
Cristóbal Sifón1? , Marcello Cacciato1 , Henk Hoekstra1 , Margot Brouwer1 ,
Edo van Uitert2,3 , Massimo Viola1 , Ivan Baldry4 , Sarah Brough5 , Michael J. I. Brown6 ,
Ami Choi7 , Simon P. Driver8,9 , Thomas Erben3 , Aniello Grado10 , Catherine Heymans7 ,
Hendrik Hildebrandt3 , Benjamin Joachimi2 , Jelte T. A. de Jong1 , Konrad Kuijken1 ,
John McFarland11 , Lance Miller12 , Reiko Nakajima3 , Nicola Napolitano, Peder Norberg13 ,
Aaron
S. G. Robotham8 , Peter Schneider3 , Gijs Verdoes Klein11
1
Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, Netherlands
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
ur Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
4 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF
5 Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
6 Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
7 Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
8 International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
9 Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK
10 INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16 80131 Napoli Italy
11 Kapteyn Astronomical Institute, University of Groningen
12 Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH
13 ICC & CEA, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
2
6 July 2015
ABSTRACT
We use the first 100 deg2 of overlap between the Kilo-Degree Survey (KiDS) and the
Galaxy And Mass Assembly (GAMA) survey to determine the galaxy halo mass of ∼10,000
spectroscopically-confirmed satellite galaxies in massive (M > 1013 h−1 M ) galaxy groups.
Separating the sample as a function of projected distance to the group centre, we jointly model
the satellites and their host groups with Navarro-Frenk-White (NFW) density profiles, fully
accounting for the data covariance. The probed satellite galaxies in these groups have total
masses log Msub /(h−1 M ) ≈ 11.7 − 12.2 consistent across group-centric distance within the
errorbars. Given their typical stellar masses, log M?,sat /(h−2 M ) ∼ 10.5, such total masses
imply stellar mass fractions of M?,sat /Msub ≈ 0.04 h−1 . The average subhalo hosting these
satellite galaxies has a mass Msub ∼ 0.015Mhost independent of host halo mass, in broad
agreement with the expectations of structure formation in a ΛCDM universe.
Key words: Gravitational lensing: weak – Galaxies: evolution, general, haloes – Cosmology:
observations, dark matter
1
INTRODUCTION
Following a hierarchical build-up, galaxy groups grow by accretion
of smaller groups and isolated galaxies. Tidal interactions tend to
transfer mass from infalling galaxies to the (new) host group, with
the former becoming group satellites. The favoured cosmological
scenario posits that galaxies are embedded in larger dark matter
haloes, with masses that largely exceed the stellar masses, a conclu?
c 2015 The Authors
sion supported by a variety of observations (see, e.g., the reviews
by Trimble 1987; Einasto 2013). Accordingly, satellite galaxies are
hosted by ‘subhaloes,’ whose masses and distribution contain information on the properties of dark matter itself (e.g., Libeskind et
al. 2013).
Because dark matter is (at least to a good approximation) dissipationless and baryons are not, it is subject to stronger tidal disruption than the baryonic component: energy losses cause baryons
to sink to the centre of the potential more efficiently than dark
matter, and therefore baryons are more resistant to tidal disrup-
Galaxy evolution within the Kilo-Degree Survey
C. Tortora, N. R. Napolitano, F. La Barbera, N. Roy, M. Radovich, F. Getman,
M. Brescia, S. Cavuoti, M. Capaccioli, G. Longo and the KiDS collaboration
Abstract The ESO Public Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried out with the VLT Survey Telescope and the OmegaCAM camera.
KiDS will scan 1500 square degrees in four optical filters (u, g, r, i). Designed to
be a weak lensing survey, it is ideal for galaxy evolution studies, thanks to the high
spatial resolution of VST, the good seeing and the photometric depth. The surface
photometry have provided with structural parameters (e.g. size and Sérsic index),
aperture and total magnitudes have been used to derive photometric redshifts from
Machine learning methods and stellar masses/luminositites from stellar population
synthesis. Our project aimed at investigating the evolution of the colour and structural properties of galaxies with mass and environment up to redshift z ∼ 0.5 and
more, to put constraints on galaxy evolution processes, as galaxy mergers.
We start from the KiDS-Data Release 2 (DR2) multi-band source catalogs, based
on source detection in the r-band images. Star/galaxy separation is based on the
CLASS STAR (star classification) and S/N (signal-to- noise ratio) parameters provided by S-Extractor [2, 12]. S-Extractor will provide with aperture photometry
and Kron-like magnitude MAG AUTO. From the original catalog of ∼ 22 millions
of sources, the star/galaxy separation leaves with a sample of ∼ 7 millions of
galaxies. To perform galaxy evolution studies and determine reliable structural parameters, the highest-quality sources have to be taken [12, 11]. Thus, we have fiC. Tortora e-mail: [email protected], N.R. Napolitano, F. La Barbera, F. Getman,
M. Brescia, S. Cavuoti
INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 - Napoli, Italy,
M. Radovich
NAF - Osservatorio Astronomico di Padova, vicolo dellOsservatorio 5, I-35122 Padova, Italy
N. Roy, M. Capaccioli, G. Longo
Department of Physics, University of Napoli Federico II, via Cinthia 9, 80126 Napoli, Italy
KiDS collaboration: http://kids.strw.leidenuniv.nl/team.php
1
Printed July 6, 2015
Dark matter halo properties of GAMA galaxy groups from 100
square degrees of KiDS weak lensing data
M. Viola1? , M. Cacciato1 , M. Brouwer1 , K. Kuijken1 , H. Hoekstra1 , P. Norberg2 ,
A.S.G. Robotham3 , E. van Uitert4,5 , M. Alpaslan12 , I.K. Baldry6 , A. Choi7 ,
J.T.A. de Jong1 , S.P. Driver3,14 , T. Erben5 , A. Grado9 , Alister W. Graham8 , C. Heymans7 ,
H. Hildebrandt5 , A.M. Hopkins13 , N. Irisarri1 , B. Joachimi4 , J. Loveday11 ,
L. Miller10 , R. Nakajima5 , P. Schneider5 , C. Sifón1 , G. Verdoes Kleijn15
1 Leiden
Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.
Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
3 ICRAR, School of Physics, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.
4 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
ur Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany.
6 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK.
7 Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK.
8 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia.
9 INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16 -80131 Napoli Italy.
10 Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH.
11 Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK.
12 NASA Ames Research Centre, N232, Moffett Field, Mountain View, CA 94035, United States.
13 Australian Astronomical Observatory, P.O. Box 915, North Ryde, NSW, Australia.
14 Scottish Universities’ Physics Alliance (SUPA), School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK.
15 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands.
2 ICC,
July 6, 2015
ABSTRACT
The Kilo-Degree Survey (KiDS) is an optical wide-field survey designed to map the matter distribution in the Universe using weak gravitational lensing. In this paper, we use these
data to measure the density profiles and masses of a sample of ∼ 1400 spectroscopically
identified galaxy groups and clusters from the Galaxy And Mass Assembly (GAMA) survey. We detect a highly significant signal (signal-to-noise-ratio ∼ 120), allowing us to study
the properties of dark matter haloes over one and a half order of magnitude in mass, from
M ∼ 1013 − 1014.5 h−1 M . We interpret the results for various subsamples of groups using
a halo model framework which accounts for the mis-centring of the Brightest Cluster Galaxy
(used as the tracer of the group centre) with respect to the centre of the group’s dark matter
halo. We find that the density profiles of the haloes are well described by an NFW profile
with concentrations that agree with predictions from numerical simulations. In addition, we
constrain scaling relations between the mass and a number of observable group properties. We
find that the mass scales with the total r-band luminosity as a power-law with slope 1.16±0.13
(1-sigma) and with the group velocity dispersion as a power-law with slope 1.89 ± 0.27 (1sigma). Finally, we demonstrate the potential of weak lensing studies of groups to discriminate
between models of baryonic feedback at group scales by comparing our results with the predictions from the Cosmo-OverWhelmingly Large Simulations (Cosmo-OWLS) project, ruling
out models without AGN feedback.
Key words: Cosmology: dark matter; Galaxies: haloes, large scale structure of the Universe;
Physical data and processes: gravitational lensing; Methods: statistical
?
[email protected]
Draft 7/2/15
CHARTING UNEXPLORED DWARF GALAXY TERRITORY WITH RR LYRAE
Mariah Baker1 , Beth Willman1
Draft 7/2/15
ABSTRACT
Observational bias against finding Milky Way (MW) dwarf galaxies at low Galactic latitudes (b <
∼
−2
20◦ ) and at low surface brightnesses (µV,0 >
∼ 29 mag arcsec ) currently limits our understanding of
the faintest limits of the galaxy luminosity function. This paper is a proof-of-concept that groups of
two or more RR Lyrae stars reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of
parameter space, with only modest contamination from interloper groups when large halo structures
are excluded. For example, a friends-of-friends (FOF) search with a linking length of 500 pc could
reveal dwarf galaxies more luminous than MV = -3.2 mag and with surface brightnesses as faint as
31 mag arcsec−2 (or even fainter, depending on RR Lyrae specific frequency). Although existing
public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include <1% of the MW halo’s
volume, a FOF search reveals two known dwarfs (Boötes I and Sextans) and two dwarf candidate
groups possibly worthy of follow-up. PanSTARRS 1 (PS1) may catalog RR Lyrae to 100 kpc which
would include ∼15% of predicted MW dwarf galaxies. Groups of PS1 RR Lyrae should therefore
reveal very low surface brightness and low Galactic latitude dwarfs within its footprint, if they exist.
With sensitivity to RR Lyrae to d >600
kpc, LSST is the only planned survey that will be both
∼
wide-field and deep enough to use RR Lyrae to definitively measure the Milky Way’s dwarf galaxy
census to extremely low surface brightnesses, and through the Galactic plane.
Subject headings: galaxies: star clusters — galaxies: dwarf — stars: distances — stars: variables:
other — techniques: photometric
1. INTRODUCTION
The Milky Way’s (MW’s) dwarf galaxy population provides a unique laboratory to study galaxies at the bottom of the galaxy luminosity function, and to quantify tensions between cosmological predictions and observations on sub-galactic scales. For example, the
discrepancy between the thousands of dwarf galaxymass dark matter halos predicted to orbit the Milky
Way (e.g. Springel et al. 2008; Garrison-Kimmel et al.
2014a) and the ∼35 dark matter dominated dwarf
galaxies observed to orbit the MW (McConnachie
2012; The DES Collaboration et al. 2015; Koposov et al.
2015) has been long referred to as the “missing satellite problem” (Klypin et al. 1999; Moore et al. 1999).
The discrepancy between the predicted and observed circular velocities of the most massive MW dwarf satellites has been coined the “too big to fail” problem
(Boylan-Kolchin et al. 2011, 2012). Most recently, increasing attention has been paid to possible differences
between the predicted and observed spatial distributions
of dwarf galaxies around the MW, M31 and other MWanalog galaxies (e.g. Kroupa et al. 2005; Pawlowski et al.
2012; Ibata et al. 2014; Phillips et al. 2015). Resolving
any of these open puzzles about the MW’s dwarf satellites, or measuring the empirical relationships needed to
understand galaxy formation at its lower limit (for example, the empirical size-luminosity-distance relationship),
requires a more complete and unbiased census of MW
dwarfs.
The current detection biases against low surface brightness and low Galactic latitude dwarfs create an incom1 Department of Astronomy, Haverford College, Haverford, PA
19041, USA, [email protected]
plete picture of the MW’s dwarf galaxy population. For
example, some theories predict the existence of extremely
low surface brightness “stealth” (Bullock et al. 2010) and
“fossil” galaxies (Bovill & Ricotti 2011) that would have
evaded detection with existing techniques. Figure 1
shows that no dwarfs are known to exist in the larger
size - lower luminosity space that such extreme dwarf
galaxies may occupy. This demographic gap may reflect
something physical about the formation of low luminosity
dwarf galaxies, or may simply reflect the detection bias
against dwarfs with V-band central surface brightnesses
fainter than ∼29 mag arcsec−2 (e.g Koposov et al. 2008;
Walsh et al. 2009; The DES Collaboration et al. 2015).
Moreover, MW dwarf galaxies are detected and studied by their resolved stellar populations. The increasing
number of foreground stars at Galactic latitudes lower
than b ∼ 20◦ therefore limits the detectability of dwarf
galaxies over a significant fraction of the MW’s volume
(Walsh et al. 2009).
RR Lyrae (RRL) stars may provide a new means to
detect previously unseen MW dwarf galaxies. RR Lyrae
are short-period variable stars found in old and metalpoor stellar populations. At least one RR Lyrae has been
found in each Milky Way dwarf galaxy searched for such
stars (see Section 3.1). There are two main categories
of RR Lyrae stars: type c RR Lyrae stars (RRc), which
pulsate in the first overtone and have periods between
0.25 and 0.4 days, and type ab RR Lyrae stars (RRab),
which pulsate in the fundamental mode and have periods between 0.4 and 0.8 days (Smith 1995; Vivas et al.
2004; Sesar et al. 2007). RR Lyrae have light curves that
exhibit distinct shapes, periods and amplitudes, making
them relatively easy to identify in time domain surveys.
RRab stars are also particularly good standard candles,
Strong lens search in the ESO public Survey
KiDS
N. R. Napolitano, G. Covone, N. Roy, C. Tortora, F. La Barbera, M. Radovich,
F. Getman, M. Capaccioli, A. Colonna, M. Paolillo, G. A. Verdoes Kleijn,
L.V.E. Koopmans and KiDS collaboration
Abstract We have started a systematic search of strong lens candidates in the ESO
public survey KiDS based on the visual inspection of massive galaxies in the redshift
range 0.1 < z < 0.5. As a pilot program we have inspected 100 sq. deg., which overlap with SDSS and where there are known lenses to use as a control sample. Taking
advantage of the superb image quality of VST/OmegaCAM, the colour information
and accurate model subtracted images, we have found 18 new lens candidates, for
which spectroscopic confirmation will be needed to confirm their lensing nature and
study the mass profile of the lensing galaxies.
1 Dark matter in galaxy cores
The dominant role of cold dark matter (CDM) in shaping structures in the Universe
is now very well constrained and understood on the largest scales, but less on the
scales of individual galaxy haloes. The assembly and evolution of CDM haloes can
be studied in detail with N-body simulations, which predict that the CDM density
profile, ρCDM (r), are well described by the so-called NFW profile with ρCDM (r) ∝
r−3 in the outer regions, and ρCDM (r) ∝ r−α , in the centre (with α = −1, [15];
α = −1.5 [14]). However, N-body simulations only follow the evolution of CDM
N.R. Napolitano e-mail: [email protected], C. Tortora, F. La Barbera, F. Getman
INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 - Napoli, Italy
G. Covone, N. Roy, M. Capaccioli, A. Colonna, M. Paolillo
Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Compl. Univ. Monte S. Angelo,
80126 - Napoli, Italy
M. Radovich
INAF – Osservatorio Astronomico di Padova-vicolo Osservatorio 5 - 35122 Padova,Italy
Gijs A. Verdoes Kleijn, L.V.E. Koopmans Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands
1
Mon. Not. R. Astron. Soc. 000, 1–?? (xxxx)
Printed 6 July 2015
Towards a census of super-compact massive galaxies in the
Kilo Degree Survey
C. Tortora1⋆ , F. La Barbera1 , N.R. Napolitano1, N. Roy1,2, M. Radovich3,
S. Cavuoti1, M. Brescia1, G. Longo2 , F. Getman1 , M. Capaccioli2, L. Grado1 ,
K. H. Kuijken4, J. T. A. de Jong4 , J. P. McFarland5
1
2
3
4
5
INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 - Napoli, Italy
Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Compl. Univ. Monte S. Angelo, 80126 - Napoli, Italy
INAF – Osservatorio Astronomico di Padova, Via Ekar, 36012 Asiago VI 0424 462032
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, the Netherlands
Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands
Accepted Received
ABSTRACT
The abundance of compact and massive early-type galaxies (ETGs) can provide significant constraints on the galaxy merging history. Thanks to the area covered, depth,
excellent spatial resolution and seeing, the ESO Public optical Kilo Degree Survey
(KiDS), carried out with the VLT Survey Telescope (VST), gives a unique opportunity to perform a complete census of the most compact galaxies in the Universe. This
paper presents a first census of compact galaxy candidates from the first 156 square
degrees of KIDS. Effective radii (Re ) in the g-, r-, and ı- bands are derived fitting
galaxy images with PSF-convolved Sérsic models, high-quality photometric redshifts,
zphot , are derived from machine learning techniques, and stellar masses, M⋆ , are calculated by fitting aperture photometry with predictions from stellar population models.
10
After the morphological star/galaxy separation, massiveness (M⋆ >
∼8 × 10 M⊙ ) and
compactness (Re <
∼ 1.5 kpc in g-, r- and ı-bands) criteria are applied, and a visual
inspection of the candidates plus near-infrared photometry from VIKING-DR1 are
used to refine our sample. The final catalog contains 92 compact systems in the redshift range z ∼ 0.2 − 0.7. This sample, to be spectroscopically confirmed, represents
the first attempt to select massive super-compact ETGs (MSCGs) in KiDS, a sample
that we expect to increase, by a factor of ten, over the total survey area (∼ 1500 sq.
deg.). Sizes of the MSCGs are significantly smaller that those of normal ETGs, at any
redshift, showing Sérsic indices which widely spread around a median value of ∼ 4.
MSCGs also present negative colour gradients, consistent with the assumption of these
systems being passively evolving. We investigate the impact of redshift systematics in
the selection, finding that, indeed, this seems a major source of contamination in our
sample. Finally, we show that the number density of MSCGs, as a function of redshift, is
mildly consistent with predictions from the Millennium Simulation for z > 0.2, while,
remarkably, no such system is found at z < 0.2.
Key words: galaxies: evolution – galaxies: general – galaxies: elliptical and lenticular,
cD.
1
INTRODUCTION
The understanding of the physical processes which drive the
galaxy mass built up and size accretion are among the most
timely topics in galaxy evolution studies. Massive early-type
⋆
galaxies (ETGs) are found to be much more compact in the
past with respect to local counterparts (Daddi et al. 2005;
Trujillo et al. 2006; Trujillo et al. 2007; van der Wel et al.
2008). At redshifts z > 2, while the massive star-forming
disks have effective radii of several kpc (Genzel et al. 2008),
the quenched spheroids (“red nuggets”) have small effective
radii of about 1 kpc. Such red nuggets are thought to be
MNRAS 000, 000–000 (0000)
External pressure-triggering of star formation in a disc galaxy: a
template for positive feedback
Rebekka Bieri1? , Yohan Dubois1 , Joseph Silk1,2,3,4 , Gary A. Mamon1
and Volker Gaibler5
1
2
3
4
5
Institut d’Astrophysique de Paris (UMR 7095: CNRS & UPMC – Sorbonne Universités), 98 bis bd Arago, F-75014 Paris, France
Laboratoire AIM-Paris-Saclay, CEA/DSM/IRFU, CNRS, Univ. Paris VII, F-91191 Gif-sur-Yvette, France
Department of Physics and Astronomy, The Johns Hopkins University Homewood Campus, Baltimore, MD 21218, USA
BIPAC, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH
Institut für Theoretische Astrophysik, Universität Heidelberg, Albert-Ueberle-Str 2, D–69120 Heidelberg, Germany
Accepted . Received ; in original form
ABSTRACT
Feedback from active galactic nuclei (AGN) has often been invoked both in simulations and
in interpreting observations for regulating star formation and quenching cooling flows in massive galaxies. AGN activity can, however, also over-pressurise the dense star-forming regions
of galaxies and thus enhance star formation, leading to a positive feedback effect. To understand this pressurisation better, we investigate the effect of an ambient external pressure on
gas fragmentation and triggering of starburst activity by means of hydrodynamical simulations. We find that moderate levels of over-pressurisation of the galaxy boost the global star
formation rate of the galaxy by an order of magnitude, turn stable discs unstable, and lead to
significant fragmentation of the gas content of the galaxy, similar to what is observed in high
redshift galaxies.
Key words: galaxies: formation — galaxies: active — methods: numerical
1
INTRODUCTION
Supermassive black holes are found at the centers of most, if not all,
massive galaxies (e.g., Magorrian et al. 1998; Hu 2008; Kormendy
et al. 2011). Throughout cosmic history, they are thought to play an
important role in regulating the baryonic mass content of massive
galaxies through feedback from Active Galactic Nuclei (AGN) by
releasing a fraction of the rest-mass accreted energy back into the
galactic gas and altering the star formation rate (SFR) in the galaxy.
AGN can exert either negative or positive feedback on their
surroundings. The former describes cases where the AGN inhibits
star formation by heating and dispersing the gas in the galaxy, while
the latter describes the possibility that an AGN may trigger star formation. Negative AGN feedback can operate in quasar-mode from
radiation at high accretion rates, or radio-mode from AGN jets at
predominantly low accretion rates (Churazov et al. 2005; Russell
et al. 2013). It is still unclear how efficiently AGN feedback delivers energy (through heating, e.g., Silk & Rees 1998) and momentum (through physical pushing, King 2003) to the galaxy’s gas
and what mode of feedback dominates. Both semi-analytical (e.g.,
Croton et al. 2006; Bower et al. 2006) and hydrodynamical cosmological simulations (e.g., Di Matteo et al. 2005, 2008; Sijacki
et al. 2007; Booth & Schaye 2009; Dubois et al. 2010) have shown
?
c 0000 The Authors
that negative AGN feedback is an important ingredient in the formation and evolution of massive galaxies, in particular in shaping
the observed high-end tail of the galaxy mass function, and the low
SFRs in massive galaxies. Moreover, observations show that cooling flows in the hot circumgalactic and intracluster media can be
suppressed by the energy transferred by AGN jets (Bˆırzan et al.
2004; Dunn et al. 2005), again negatively impacting star formation.
Although AGN feedback has been extensively studied in observations and through cosmological simulations, the impact on the
host galaxy and the precise mechanism of the communication of the
AGN with the galaxy’s interstellar medium (ISM) is far from being understood. It is not clear why jet feedback, which is thought
to heat cold gas, should have a similar effect on the multi-phase
ISM. It has been argued that a jet that propagates through an inhomogeneous ISM may also trigger or enhance star formation in
a galaxy (i.e., positive feedback). Begelman & Cioffi (1989) and
Rees (1989) proposed that the radio jet activity triggers star formation and might serve as an explanation for the alignment of radio and optical structures in high redshift radio galaxies. Radio jetinduced star formation has also been considered as a source powering luminous starbursts (Silk 2005). Ishibashi & Fabian (2012)
provide a theoretical framework linking AGN feedback triggering
of star formation in the host galaxy to the oversized evolution of
massive galaxies over cosmic time. Furthermore, negative and positive feedback are not necessarily contradictory (Silk 2013; Zubo-
Transient Astrophysics with the Square Kilometre
Array
Rob Fender∗
Oxford Astrophysics
Adam Stewart
Oxford Astrophysics
Jean-Pierre Macquart
ICRAR/Curtin
Immacolata Donnarumma
IAPS/INAF Rome
Tara Murphy
University of Sydney
Adam Deller
ASTRON
Zsolt Paragi
Joint Institute for VLBI in Europe
Shami Chatterjee
Cornell
This chapter provides an overview of the possibilities for transient and variable-source astrophysics with the Square Kilometre Array. While subsequent chapters focus on the astrophysics of
individual events, we focus on the broader picture, and how to maximise the science coming from
the telescope. The SKA as currently designed will be a fantastic and ground-breaking facility for
radio transient studies, but the scientifc yield will be dramatically increased by the addition of (i)
near-real-time commensal searches of data streams for events, and (ii) on occasion, rapid robotic
response to Target-of-Opprtunity style triggers.
Advancing Astrophysics with the Square Kilometre Array
June 8-13, 2014
Giardini Naxos, Italy
∗ Speaker.
c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
http://pos.sissa.it/
Rob Fender
SKA Transients
1. Introduction
Radio transients are both the sites and signatures of the most extreme phenomena in our Universe: e.g. exploding stars, compact object mergers, black holes and ultra-relativistic flows. They
also have the potential to act as probes of the intervening medium on all scales, up to and including cosmological distances. As such they are invaluable probes for subjects as diverse as stellar
evolution, relativistic astrophysics and cosmology.
The majority of such transients can be broadly divided into:
• Incoherent synchrotron events, which are associated with all explosive kinetic feedback
and particle acceleration in the Universe, from relativistic jets to supernova explosions. Such
emission is limited, in a steady state, to a brightness temperature of TB ≤ 1012 K, and therefore
the most luminous events (observable at cosmological distances) vary rather slowly (days to
years).
This timescale is often longer than that of a typical observation, and variability is usually
determined by comparing images made at different epochs. As noted below, synchrotron
flares can generally be used as a measure of the kinetic feedback associated with an event,
and as such allow us to understand the flow of energy in the most extreme astrophysical
environments.
• Coherent bursts, which can achieve much higher brightness temperature (up to at least
1030 K) and hence can be of very short durations. Such events can be intrinsically much
shorter than the timescales imposed by scattering and dispersion in the intervening interstellar and intergalactic medium, and so can be used as unique probes of these environments.
These events are generally observed in ‘pulsar mode’ beamformed data.
The correspondence between emission mechansim and observing modes is not exact, however,
and very fast imaging (on millisecond timescales) and strong scattering can blur the boundaries
between the two. Furthermore, while the above constitute the majority of radio transients, strong
variability can also be observed associated with thermal emission (e.g. novae), gravitational lensing
and scattering, all of which can be picked up by the revised approaches outlined here. In all
cases, transient science benefits greatly from monitoring the sky frequently. As such, commensal
observing alongside other projects is a powerful strategy.
1.1 Examples of important science associated with radio variables
There are many examples of unexepected and/or extreme astrophysical phenomena being revealed or better understood via observations of radio variability. These include:
• Radio bursts from Jupiter (Burke & Franklin 1955).
• The discovery of neutron stars via their pulsed radio emission (Hewish et al. 1968).
• Discovery of apparent superluminal motion in relativistic jets in variable extragalactic (Cohen et al. 1971) and Galactic (‘microquasars’, Mirabel & Rodriguez 1994) sources (black
holes).
2
DUSTY GALAXIES AND THE DEGENERACY BETWEEN THEIR DUST DISTRIBUTIONS AND THE
ATTENUATION FORMULA
Kyle Penner1 , Mark Dickinson2 , Benjamin Weiner3 , Hanae Inami2 , Jeyhan Kartaltepe2 , Janine Pforr4 ,
Hooshang Nayyeri5 , Susan Kassin6 , Casey Papovich7 , and Alexandra Pope8
ABSTRACT
Do spatial distributions of dust grains in galaxies have typical forms, as do spatial distributions
of stars? We investigate whether or not the distributions resemble uniform foreground screens, as
commonly assumed by the high-redshift galaxy community. We use rest-frame infrared, ultraviolet,
and Hα line luminosities of dust-poor and dusty galaxies at z ∼ 0 and z ∼ 1 to compare measured Hα
escape fractions with those predicted by the Calzetti attenuation formula. The predictions, based on
UV escape fractions, overestimate the measured Hα escape fractions for all samples. The interpretation
of this result for dust-poor z ∼ 0 galaxies is that regions with ionizing stars have more dust than
regions with nonionizing UV-emitting stars. Dust distributions for these galaxies are nonuniform. The
interpretation of the overestimates for dusty galaxies at both redshifts is less clear. If the attenuation
formula is inapplicable to these galaxies, perhaps the disagreements are unphysical; perhaps dust
distributions in these galaxies are uniform. If the attenuation formula does apply, then dusty galaxies
have nonuniform dust distributions; the distributions are more uniform than they are in dust-poor
galaxies. A broad range of Hα escape fractions at a given UV escape fraction for z ∼ 1 dusty galaxies,
if real, indicates diverse dust morphologies and the implausibility of the screen assumption.
1. INTRODUCTION
A galaxy’s morphology is often defined as its spatial
distribution of stars. Rest-frame optical images of galaxies show diverse arrangements of their stellar components: smooth ellipticals, tightly-wound spirals, clumpy
disks, barred disks, train-wrecks, fuzzballs, and whatever
Willman 1 is (Willman et al. 2005, 2011). Little more
than spatial extent is known about distributions of dust
in dusty galaxies (Younger et al. 2010; D´ıaz-Santos et al.
2010; Simpson et al. 2015). Yet spatial distributions
of all galactic components have intrinsic value. If we
want to learn about galaxies, morphology should be a
wavelength-independent word. Surprises surely abound:
for example, galaxies at z ∼ 0.3 have dust out to several Mpc. Their average dust mass profile is a constant
fraction of their average halo mass profile (Ménard et al.
2010, see also Zaritsky 1994).
Dust attenuates a galaxy’s intrinsic luminosity at ultraviolet (UV) and optical wavelengths. To recover an
intrinsic luminosity from an emergent luminosity we often assume that dust grains are distributed as a uniformly thick screen between us and the galaxy’s stars
[email protected]
1 Laboratoire AIM, DSM/Irfu/SAp, CEA-Saclay, 91191 Gifsur-Yvette, France
2 National Optical Astronomy Observatory, Tucson, AZ
85719, USA
3 Department of Astronomy, University of Arizona, Tucson,
AZ 85721, USA
4 Laboratoire d’Astrophysique de Marseille, Aix Marseille Universit´
e, 13388 Marseille cedex 13, France
5 Department of Physics & Astronomy, University of California Irvine, Irvine, CA 92697, USA
6 Space Telescope Science Institute, Baltimore, MD 21218,
USA
7 Department of Physics & Astronomy, Texas A&M University, College Station, TX 77843, USA
8 Department of Astronomy, University of Massachusetts
Amherst, Amherst, MA 01003, USA
(Calzetti et al. 1994). From the uniform screen assumption follows an attenuation formula that depends solely
on wavelength.
An attenuation formula is denoted k(λ) or k ′ (λ). It
differs from an extinction formula, which is independent
of the spatial distribution of dust. When we derive an
extinction formula, it is valid for the small area surrounding a star in the Milky Way, not the large area containing
a mixture of stars in distant galaxies.
If we write this equation:
Lemergent/Lintrinsic = 10−A/2.5 = 10−E(B−V)k(λ)/2.5
we assume a uniform screen not because k(λ) is solely dependent on wavelength, but because Lemergent/Lintrinsic
is solely dependent on wavelength. In an illustrative
sense Lemergent/Lintrinsic is an attenuation formula convolved with a spatial distribution of dust.
The uniform screen assumption is unrealistic. A lowredshift galaxy with a high emergent UV luminosity and
a low IR luminosity—a dust-poor galaxy—has a high
UV escape fraction. The Calzetti et al. (2000) formula
predicts its Hα escape fraction; the prediction overestimates the measured Hα escape fraction. The galaxy’s
regions with > 10 M⊙ stars, which ionize gas, have more
dust than its regions with less massive stars that emit in
the UV and leave the gas unionized (Calzetti et al. 1994;
Calzetti 1997b). While the discrepancy in amounts of
dust does not invalidate the screen part of our assumption, it does invalidate the uniform part.
The screen part of our assumption is suspect, but
it may be a good approximation to the truth—the alternative assumption being that dust mixes with stars.
Liu et al. (2013) study the dust distribution of M83 at a
spatial scale of 6 pc. They divide M83 into a center and
outskirts. For each part, they calculate the percentage of
regions with ionizing stars mixed with dust. Only in the
Printed 6 July 2015
First discoveries of z ∼ 6 quasars with the Kilo Degree
Survey and VISTA Kilo-Degree Infrared Galaxy survey⋆
B. P. Venemans,1† G. A. Verdoes Kleijn,2 J. Mwebaze,2 E. A. Valentijn,2
E. Ba˜
nados,1 R. Decarli,1 J. T. A. de Jong,3 J. R. Findlay,4 K. H. Kuijken,3
F. La Barbera,5 J. P. McFarland,2 R. G. McMahon,6,7 N. Napolitano,5
G. Sikkema2 and W. J. Sutherland8
1 Max-Planck
Institute for Astronomy, K¨
onigstuhl 17, D-69117 Heidelberg, Germany
Astronomical Institute, University of Groningen, The Netherlands
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
Astronomical Observatory of Capodimonte - INAF, via Moiariello 16, I-80131, Napoli, Italy
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, London, E1 4NS, UK
2 Kapteyn
3
4
5
6
7
8
Accepted . Received ; in original form
ABSTRACT
We present the results of our first year of quasar search in the on-going ESO public Kilo
Degree Survey (KiDS) and VISTA Kilo-Degree Infrared Galaxy (VIKING) surveys.
These surveys go up to 2 magnitudes fainter than other wide-field imaging surveys
that uncovered predominantly very luminous quasars at z ∼ 6. This allows us to
probe a more common, fainter population of z ∼ 6 quasars. From this first set of
combined survey catalogues covering ∼250 deg2 we selected point sources down to
ZAB = 22 that had a very red i − Z (i − Z > 2.2) colour. After follow-up imaging
and spectroscopy, we discovered four new quasars in the redshift range 5.8 < z < 6.0.
The absolute magnitudes at a rest-frame wavelength of 1450 ˚
A are between −26.6 <
M1450 < −24.4, confirming that we can find quasars fainter than M ∗ , which at z = 6
has been estimated to be between M ∗ = −25.1 and M ∗ = −27.6. The discovery
of 4 quasars in 250 deg2 of survey data is consistent with predictions based on the
z ∼ 6 quasar luminosity function. We discuss various ways to push the candidate
selection to fainter magnitudes and we expect to find about 30 new quasars down to
an absolute magnitude of M1450 = −24. Studying this homogeneously selected faint
quasar population will be important to gain insight into the onset of the co-evolution
of the black holes and their stellar hosts.
Key words: cosmology: observations — galaxies: active — galaxies: quasars: general
1
INTRODUCTION
Supermassive black holes (SMBHs) are inferred to reside at
the nuclei of most (if not all) massive galaxies. In present day
galaxies, tight correlations are observed between the black
hole mass and global host parameters over at least 3 orders of
magnitude in black hole mass (see e.g Kormendy & Ho 2013,
⋆
Based on observations collected at the European Southern
Observatory, Chile, programmes 177.A-3016, 179.A-2004, 089.A0290, 089.A-0596, 090.A-0383, 090.A-0642 and 091.A-0421.
† E-mail: [email protected]
for a recent review). Going back in time, the global history
of star formation and of radiative mass accretion onto black
holes display similar behavior as a function of time, peaking at redshifts ∼ 2 − 3 (Heckman et al. 2004; for a review
see Madau & Dickinson 2014 and references therein). Determining the physical processes that govern the symbiotic
relationship between SMBHs and their host galaxies is an
outstanding challenge in astrophysics. Studying the SMBHs
and stellar hosts of high redshift (z > 5.7) quasars is one of
the most direct ways to obtain insight on the onset of this
relation in the first Gyr after the Big Bang. They provide
fundamental constraints on the formation and growth of the
Prime Focus Spectrograph for the Subaru telescope:
massively multiplexed optical and near-infrared fiber spectrograph
Hajime Sugai*a, Naoyuki Tamuraa, Hiroshi Karojia, Atsushi Shimonoa, Naruhisa Takatob, Masahiko
Kimurac, Youichi Ohyamac, Akitoshi Uedad, Hrand Aghazariane, Marcio Vital de Arrudaf, Robert H.
Barkhouserg, Charles L. Bennettg, Steve Bickertona, Alexandre Bozierh, David F. Braune, Khanh Buii,
Christopher M. Capocasalee, Michael A. Carrj, Bruno Castilhof, Yin-Chang Changc, Hsin-Yo Chenc,
Richard C.Y. Chouc, Olivia R. Dawsone, Richard G. Dekanyk, Eric M. Eke, Richard S. Ellisi, Robin J.
Englishe, Didier Ferrandh, Décio Ferreiraf, Charles D. Fishere, Mirek Golebiowskig, James E. Gunnj,
Murdock Hartg, Timothy M. Heckmang, Paul T. P. Hoc, Stephen Hopeg, Larry E. Hovlande, Shu-Fu
Hsuc, Yen-Shan Huc, Pin Jie Huangc, Marc Jaqueth, Jennifer E. Karrc, Jason G. Kempenaare,
Matthew E. Kinge, Olivier Le Fèvreh, David Le Mignanth, Hung-Hsu Lingc, Craig Loomisj, Robert H.
Luptonj, Fabrice Madech, Peter Maoi, Lucas Souza Marraraf, Brice Ménardg, Chaz Morantze, Hitoshi
Murayamaa, Graham J. Murrayl, Antonio Cesar de Oliveiraf, Claudia Mendes de Oliveiram, Ligia
Souza de Oliveiraf, Joe D. Orndorffg, Rodrigo de Paiva Vilaçaf, Eamon J. Partose, Sandrine Pascalh,
Thomas Pegot-Ogierh, Daniel J. Reileyi, Reed Riddlei, Leandro Santosf, Jesulino Bispo dos Santosf,
Mark A. Schwocherte, Michael D. Seifferte,i, Stephen A. Smeeg, Roger M. Smithk, Ronald E.
Steinkrause, Laerte Sodré Jrm, David N. Spergelj, Christian Suraceh, Laurence Tresseh, Clément
Vidalh, Sebastien Vivesh, Shiang-Yu Wangc, Chih-Yi Wenc, Amy C. Wue, Rosie Wyseg, Chi-Hung
Yanc
a
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 51-5, Kashiwanoha, Kashiwa, 277-8583, Japan;
b
Subaru Telescope, National Astronomical Observatory of Japan, 650 North Aòhoku Pl., Hilo,
Hawaii, 96720, USA;
c
Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei, Taiwan;
d
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan;
e
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA;
f
Laboratório Nacional de Astrofisica, MCTI, Rua Estados Unidos, 154, Bairro das Nações, Itajubá,
MG, Brazil;
g
Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street,
Baltimore, MD 21218, USA;
h
Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille) UMR 7326,
13388, Marseille, France;
i
Astronomy Department, California Institute of Technology, 1200 East California Blvd, Pasadena,
CA 91125, USA;
j
Department of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544, USA;
k
Caltech Optical Observatories, 1201 East California Blvd., Pasadena, CA 91125 USA;
l
Centre For Advanced Instrumentation, Durham University, Physics Dept, Rochester Bldg, South
Road, Durham DH1 3LE, UK;
m
Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de São Paulo, Rua do Matão, 1226 - Cidade
Universitária - 05508-090, Brazil
* [email protected]; phone 81 4 7136-6551; fax 81 4 7136-6576;
http://member.ipmu.jp/hajime.sugai/
1
ABSTRACT
The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394
science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The
wide wavelength coverage from 0.38 μm to 1.26 μm, with a resolving power of 3000,
simultaneously strengthens its ability to target three main survey programs: cosmology, galactic
archaeology and galaxy/AGN evolution. A medium resolution mode with a resolving power of 5000
for 0.71 μm to 0.89 μm will also be available by simply exchanging dispersers. We highlight some
of the technological aspects of the design. To transform the telescope focal ratio, a broad-band
coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part
of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is
selected for the fiber-positioner and fiber-slit components, minimizing the effects of fiber
movements and fiber bending. Fiber positioning will be performed by a positioner consisting of two
stages of piezo-electric rotary motors. The positions of these motors are measured by taking an
image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain
container; the fibers are placed in the proper location by iteratively measuring and then adjusting the
positions of the motors. Target light reaches one of the four identical fast-Schmidt spectrograph
modules, each with three arms. The PFS project has passed several project-wide design reviews and
is now in the construction phase.
Keywords: spectrographs, fiber applications, optical systems, infrared systems, astronomy
1. INTRODUCTION
The Prime Focus Spectrograph (PFS)1 is an optical/near-infrared multifiber spectrograph with 2394
science fibers (Fig. 1), and is scheduled to be mounted on the Subaru 8.2-m telescope. The position
of each fiber is controlled by a fiber positioner, called “Cobra,” consisting of two-staged piezoelectric rotary squiggle motors. Each fiber positioner unit samples a circular patrol region of 9.5-mm
diameter at the telescope focal plane, and these positioners are arrayed into a hexagonal pattern with
8-mm central distances between adjacent positioners. The whole array extends to a single hexagonal
field of view (FOV) whose effective diameter is 1.3 deg. The hexagonal FOV has an advantage for
efficient tiling of survey areas, compared with e.g., a circular FOV. The fibers are divided into four
groups, each of which enters one of the four spectrograph modules in a slit comprising 600 or 597
science fibers in a single row. Each spectrograph module has three color arms to cover a wide
wavelength region from 0.38 μm to 1.26 μm, with a mean resolving power λ∕δλ of 3000. This
resolution is optimal for spectroscopic surveys of fainter galaxies and stars, targeting cosmology,
Galactic archaeology and galaxy/AGN evolution. To simplify the design of the spectrograph, the
F∕2.2 beam from the telescope wide field corrector (WFC) is increased to F∕2.8 by a microlens
bonded to each fiber. A double Schmidt design is used for the spectrograph modules, which are F∕2.5
at the fiber side and F∕1.09 at the detector side. These apertures are slightly larger than the nominal
F∕#, allowing for some focal ratio degradation (FRD) in the fiber. A medium spectral resolution
mode with resolving power of 5000 for the red arms (0.71 μm to 0.89 μm) is also included. This
addition particularly benefits important Galactic archaeology studies. This new mode is realized by
2
DISK-STABILITY CONSTRAINTS ON THE NUMBER OF ARMS IN SPIRAL GALAXIES
Elena D’Onghia1
Department of Astronomy, University of Wisconsin, 475 N. Charter Street, Madison, WI 53076, USA
ABSTRACT
A model based on disk-stability criteria to determine the number of spiral arms of a general disk
galaxy with an exponential disk, a bulge and a dark halo described by a Hernquist model is presented.
The multifold rotational symmetry of the spiral structure can be evaluated analytically once the
structural properties of a galaxy, such as the circular speed curve, and the disk surface brightness,
are known. By changing the disk mass, these models are aimed at varying the critical length scale
parameter of the disk and lead to a different spiral morphology in agreement with prior models.
Previous studies based on the swing amplification and disk stability have been applied to constrain
the mass-to-light ratio in disk galaxies. This formalism provides an analytic expression to estimate
the number of arms expected by swing amplification making its application straight-forward to large
surveys. It can be applied to predict the number of arms in the Milky Way as a function of radius and
to constrain the mass-to-light ratio in disk galaxies for which photometric and kinematic measurements
are available, like in the DiskMass survey. Hence, the halo contribution to the total mass in the inner
parts of disk galaxies can be inferred in light of the ongoing and forthcoming surveys.
Subject headings: Galaxy: disk - Galaxy: evolution - galaxies: kinematics and dynamics - stars:
kinematics and dynamics
1. INTRODUCTION
A long-standing problem in understanding the dynamics of disk galaxies is the uncertainty of the fractional
contribution of the dark halo to the total mass of the
galaxy within the optical radius of the stellar disk. While
galaxies are believed to be dark matter dominated in
their outer parts, at radii much larger than the optical
radius, the assessment of the relative fraction of baryons
and dark matter in the inner part of disk galaxies is still
debated (e.g. Courteau et al. 2014).
The circular-speed curve constrains the total mass distribution at all radii, and the disk contribution is usually
estimated by combining the light profile with an estimate
of the mass-to-light ratio of the disk. The difference between the total rotation curve and the circular velocity
of the disk is assumed to be due to the presence of dark
matter (Bosma 1978; Carignan & Freeman 1985). Because the mass-to-light ratio of the disk is uncertain, the
relative contributions of the disk and halo are poorly determined.
Attempts to constrain the mass-to-light ratio come
from studies of stellar populations (e.g. Conroy 2013),
and comparisons between dynamical and stellar population mass estimates (Bell & de Jong 2001). However,
these models depend on assumptions made about the
IMF and stellar ages, leading to rather large uncertainties in the disk contribution. A maximum disk hypothesis
for the mass decomposition has been suggested for disk
galaxies (van Albada et al. 1985). It assumes that the
total rotation curve of disk galaxies out to the optical
radius can be fitted by models that assume a mass-tolight ratio of the disk independent of radius and with
no need for dark matter in the inner parts (maximum
disk). A typical maximum disk contributes more than
Electronic address: e-mail:[email protected]
1 Alfred P. Sloan Fellow
80% of the total circular speed at 2.2 Rd , where Rd is
the scale length of the exponential disk. In terms of disk
mass fraction the maximum disk hypothesis requires a
disk contribution of fd (2.2 Rd )=(Vdisk /Vtot )2 >0.7.
Additional arguments have been made to constrain
the halo and disk contribution to the total gravitational
field via different techniques. In particular, studies on
Tully-Fisher residuals applied to bright disk galaxies suggest that on average disk galaxies are sub-maximal with
fd (2.2 Rd )=0.25-0.5 (Courteau & Rix 1999). Another
approach assumes that the peak circular velocity of the
stellar disk, measured at R=2.2 Rd , is related to the vertical velocity dispersion, and the scale height through
some scaling relations (van der Kruit 1988). More recently, Bershady et al. (2010) applied the velocity dispersion technique to a sample of 46 nearly face-on galaxies finding that the disk fraction of their sample ranges
between 0.16 and 0.5. A similar method has been recently applied to the Milky Way. The analysis includes
the ability to disentangle stellar populations and results
in a low estimate of the disk scale length and a correspondingly high disk fraction of 0.69±0.07 (Bovy & Rix
2013)(hereafter BR13).
The study here presents a model based on swing amplification (Toomre 1981) to compute analytically the
azimuthal wavenumber at each radius that is likely to
experience the strongest growth to determine the number of spiral arms expected at that radius(Athanassoula
1988; Toomre & Kalnajs 1991; Bosma 1999; Fuchs &
Athanassoula 2005). For a sample of grand design spirals, Athanassoula et al. (1987), hereafter ABP87, applied the swing amplification formalism in an attempt
to constrain the mass-to-light ratio of dozens of disks.
ABP87 study is of considerable observational interest,
since it treated the disk stars and gas as distinct components, and computed their contribution of the total rotation curve using directly their observed projected sur-

Abstract - UChicago High Energy Physics

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