Vladimir Avila-Reese Instituto de Astronomía, UNAM

Controversies of LCDM on
small scales
Vladimir Avila-Reese
Instituto de Astronomía, UNAM
Cosmology on the Beach 2014, Los Cabos
Contents
• LCDM, the simplest cosmological scenario
• What are the LCDM controversies on small
scales?
• Is baryon physics behind the controversies?
• Relax assumptions: dark matter alternatives
• Key challenges
• Concluding remarks
...To take home
A better understanding of the gastrophysics and the
baryon-dark matter interaction is necessary to
acquire before claiming for issues at small scales
Key tests for LCDM: discovery of the predicted
abundant substructure and the halo triaxiality
The dark sector is likely much more complex than
our simple models, with its own (self)interactions and
particle zoo.
SIDM is promising.
The emergence of the CDM scenario
Critical issues in Astronomy/Cosmology during decades
1. Dynamical inferences from
observations (under GR and
Newton dynamics) in conflict with
luminous mass distributions
lack
of gravitational forces
2. Galaxy disks are dynamically
unstable, but today ~80% of
galaxies are of disk type
3. S i l k d a m p i n g o f b a r y o n i c
fluctuations in the hot universe
erases perturbations of scales
<1013 Msun --> no seeds for galaxy
formation!
‣ kinematics of galaxies and
clusters (Vrot(r), σ(r), etc.) +
‣ cosmological determinations of
baryonic and dynamical mass
densities: Ωb<<Ωdyn +
‣ gravitational lensing.
The emergence of the CDM scenario
Critical issues in Astronomy/Cosmology during decades
1. Dynamical inferences from
observations (under GR and
Newton dynamics) in conflict with
luminous mass distributions
lack
of gravitational forces
‣ kinematics of galaxies and
clusters (Vrot(r), σ(r), etc.) +
‣ cosmological determinations of
baryonic and dynamical mass
densities: Ωb<<Ωdyn +
‣ gravitational lensing.
2. Galaxy disks are dynamically
unstable, but today ~80% of
galaxies are of disk type
Dominant fraction of nonbaryonic DARK MATTER.
3. S i l k d a m p i n g o f b a r y o n i c
fluctuations in the hot universe
erases perturbations of scales
<1013 Msun --> no seeds for galaxy
formation!
DM should be COLD to avoid freestreaming (WARM DM could work
well too).
Besides in Particle Physics
the dark sector is required
Lacking forces in galaxies/clusters + dynamical stability of
disks + issues of Ωb<<Ωdyn & Silk damping + necessity of a
dark sector in the SM of particles (best candidates: WIMPs)
Cold Dark matter and Λ
•If Ωtot=1 and h=0.7, T<12 Gyr
•Observations: Ωdyn~0.3 but first peak
in the CMBR anisotropies Ωtot=1
•Accelerated expansion (SN Ia,
GRBs, BAOs)
(Λ)CDM: A cosmological and cosmogonical scenario developed
iteratively on the basis of astronomical observations and within the
context of the standard physical theories and models.
Fundaments and predictions
Homogeneous universe
Cosmology
•Big Bang Theory (GR + cosmological
principle + SM of particle physics)
•Inflationary model (GR +quantum field
theory + supersymmetry?)
------------------------------------------------------------
Consistent with the BBT and Inflation
predictions:
1) Redshift of galaxies (expansion), 2)
abundances of light elements (Ωb<<1!),
3)CMBR, 4) Flat geometyr Ωtot=1.
Inhomogeneities
Cosmogony
•Gravitational paradigm + relativistic hot
plasma thermo-hydro-dynamics (Silk
damping!)
•Gravitational collapse and Galaxy
Formation (non-linear and complex
gastrophysics, yet in diapers)
-------------------------------------------------------Amazing consistency with:
Large scale structure (local and at high z)
LSS from N-body pure DM simulations
Using galaxies as tracers of halos
real
simulated
The gravitational (collisionless )
non-linear evolution produces the
cosmic web of walls, filaments and
voids as observed
Springel+06
Large scale structure (local and at high z)
LSS from N-body pure DM simulations
galaxies as
tracers of halos of LSS
ExcellentUsing
agreement
w/observations
at a quantitative level (correlation functions,
power spectrum, etc.)
real
simulated
The gravitational (collisionless )
non-linear evolution produces the
cosmic web of walls, filaments and
voids as observed
Springel+06
Large scale structure (local and at high z)
LSS from N-body pure DM simulations
galaxies as
tracers of halos of LSS
ExcellentUsing
agreement
w/observations
at a quantitative level (correlation functions,
power spectrum, etc.)
real
simulated
The gravitational (collisionless )
non-linear evolution produces the
cosmic web of walls, filaments and
voids as observed
Springel+06
Large scale structure (local and at high z)
LSS from N-body pure DM simulations
galaxies as
tracers of halos of LSS
ExcellentUsing
agreement
w/observations
at a quantitative level (correlation functions,
power spectrum, etc.)
real
simulated
The gravitational (collisionless )
non-linear evolution produces the
cosmic web of walls, filaments and
voids as observed
Springel+06
Large scale structure (local and at high z)
LSS from N-body pure DM simulations
The gravitational (collisionless )
non-linear evolution produces the
cosmic web of walls, filaments and
voids as observed
Fully consistent with the observed CMBR anisotropies
ΛCDM scenario
CDM halo mass function = observed cluster/group mass function
At difference of galaxies, the halo masses of clusters can be measured directly
(velocity dispersions, hot gas dynamics, lensing)
LCDM halo
mass functions
Vikhlinin+09
Lambda is necessary!
The Tully-Fisher relation: Vm vs M
Page 5 of 13
Firmani & Avila-Reese 00; Avila-Reese+ 08
an imprint of the ~flat region of the power spectrum of density perturbations
stellar
stellar+gas
models
models
Ms∝Vm, m≈3.3, is a consequence of the Mh ∝V3.2
cosmological relation (Avila-Reese+ 99), and this is due
to the PS flattening:
σ ≈ const → <ρh>≈ const
<ρh>∝ Mh/r3 ∝const
V ∝ (GM/r)1/2
Mh ∝ V 3
The TFR is an imprint of the primordial power
spectrum of density fluctuations
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From cosmological simulations
Figure 1. Comparison of simulated and observed relations between galaxy stellar mass and rotation velocity for disc galaxies
(i.e., the ‘Tully-Fisher’ relation) at z = 0. The top panel shows
(Λ)CDM, the simplest case: A cosmological and cosmogonical
scenario built up iteratively on the basis of astronomical observations and
within the context of the standard physical theories and models.
Fundaments and predictions
Homogeneous universe
Cosmology
•Big Bang Theory (GR + cosmological
principle + SM of particle physics)
•Inflationary model (GR +quantum field
theory + supersymmetry?)
------------------------------------------------------------
Consistent with the BBT and Inflation
predictions:
1) Redshift of galaxies (expansion), 2)
abundances of light elements (Ωb<<1!),
3)CMBR, 4) Flat geometyr Ωtot=1.
Inhomogeneities
Cosmogony
•Gravitational paradigm + hot plasma
relativistic thermo-hydro-dynamics (Silk
damping!)
•Gravitational collapse and Galaxy
Formation (non-linear and complex
gastrophysics, yet in diapers)
-------------------------------------------------------Amazing consistency with: 1) Observed
LSS, 2) CMBR anisotropies, 3) Cluster
mass function, 4) Tully-Fisher relation,
5) Galaxy properties and correlations?,
satellite distributions?, inner structure and
dynamics of galaxies? <-- potential
small scale issues!
LCDM cosmogony: *Gaussian density perturbations, *scale invariant PS
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
It’s the simplest case from the point of view of cosmic structure formation
What about
smaller
scales? *scale invariant PS
LCDM cosmogony:
*Gaussian
density perturbations,
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
It’s the simplest case from the point of view of cosmic structure formation
Dwarf galaxies
Gravitational
lensing
105 M!
108 M! 1011 M! 1014 M!
WDM ?
1M
105 Ms
Friday,sAugust 21, 2009
HDM X
108 Ms
1012 Ms
1016 Ms
1020Ms
What about
smaller
scales? *scale invariant PS
LCDM cosmogony:
*Gaussian
density perturbations,
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
It’s the simplest case from the point of view of cosmic structure formation
Dwarf galaxies
!
l
u
f
s
s
e
c
c
Gravitational
lensing
105 M!
108 M! 1011 M! 1014 M!
WDM ?
1M
105 Ms
Friday,sAugust 21, 2009
Su
HDM X
108 Ms
1012 Ms
1016 Ms
1020Ms
What about
smaller
scales? *scale invariant PS
LCDM cosmogony:
*Gaussian
density perturbations,
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
It’s the simplest case from the point of view of cosmic structure formation
Dwarf galaxies
!
l
u
f
s
s
e
c
c
Gravitational
lensing
Strong non-linear
evolution + strong
effects of
105 M dissipative
108 M 1011 M
baryons
!
!
WDM ?
1M
105 Ms
Friday,sAugust 21, 2009
!
1014 M!
Su
HDM X
108 Ms
1012 Ms
1016 Ms
1020Ms
Genetic code for cosmic structure formation: scales
53
M. Kuhlen et al. / Dark Universe 1 (2012) 50–93
Stangexpansion effect
From Kuhlen+12
Primordial PS
from inflation!
~synchronous
collapse of
small scales
1. D2(k) % 4p(k/2p)3P(k), the linear power spectrum of density fluctuations at z = 0. The solid line is the canonical cold DM
del with an Eisenstein and Hu [11] transfer function. The dashed line is a thermal relic warm DM model with mWDM = 8 keV
Genetic code for cosmic structure formation: scales
large
intermediate
small
53
M. Kuhlen et al. / Dark Universe 1 (2012) 50–93
linear, H-Z shape
quasi-linear, bend strongly non-linear, ~flat PS
Stangexpansion effect
From Kuhlen+12
Primordial PS
from inflation!
~synchronous
collapse of
small scales
1. D2(k) % 4p(k/2p)3P(k), the linear power spectrum of density fluctuations at z = 0. The solid line is the canonical cold DM
del with an Eisenstein and Hu [11] transfer function. The dashed line is a thermal relic warm DM model with mWDM = 8 keV
Contents
• LCDM, the simplest cosmological scenario
• What are the LCDM controversies on small
scales?
• Is baryon physics behind the controversies?
• Relax assumptions: dark matter alternatives
• Key challenges
• Concluding remarks
1. The cusp-core problem
Controversy since late
90’s. Observations
improved, the problem
remains but less stringent
(e.g., de Blok+05,08; Strigari
+08; Kuzio de Naray+08,11;
Oh+11; Salucci+12)
RCs of DM-dominated
galaxies (LSBs & dwarfs)
are supposed to trace
the halo gravitational
potential
LSB galaxies
e.g., Firmani+01, MNRAS
...... NFW before adiabatic contraction
0
5
R (kpc)
10
0
2
4
R (kpc)
6
8
10
0
5
10
R (kpc)
However: 1) If the disk is not axisymmetric/non-circular motions (e.g., bar) and
there are pressure gradients: Vc ≠ Vφ (Vφ accounts for other contributions!)
2) If the halos is triaxial, Vφ is larger and the central mass is underestimated
Valenzuela+2007: Simulated Barred Dwarf Irregular Galaxy: Realistically Including all the
kinematis and pressure effects. Cuspy Halo. 2% Mdisk/Mhalo . 60 pc resolution.
Stellar Component
total
Vφ by stars.
Vφ by gas.
halo
disk
Gas Component (T<15000 K)
gas
However: 1) If the disk is not axisymmetric/non-circular motions (e.g., bar) and
there are pressure gradients: Vc ≠ Vφ (Vφ accounts for other contributions!)
2) If the halos is triaxial, Vφ is larger and the central mass is underestimated
Valenzuela+2007: Simulated Barred Dwarf Irregular Galaxy: Realistically Including all the
kinematis and pressure effects. Cuspy Halo. 2% Mdisk/Mhalo . 60 pc resolution.
Stellar Component
total
Vφ by stars.
Vφ by gas.
Gas Component (T<15000 K)
interpreted as a core
but there is not a core
halo
disk
gas
Besides deviations from axisymmetry and circular motions,
•the degeneracies (e.g., between M/L ratio and core slope) are too strong to
make a constraint by the kinematics alone: stellar pop. synthesis helps.
•gaseous and stellar kinematics are often different, giving different constraints
The Astrophysical Journal, 745:92 (17pp), 2012 January 20
Integral field spectroscopy
(2D stellar/gaseous maps)
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E
Example:
NGC2974 -the prototype of
cored dwarf galaxy (Simons+03, Ha
kinematics)
VIRUS-P data + Jeans Anisotropic
Model + Stellar Pop Synthesis
(Adams+12, ApJ)
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Data on NGC 2976
Adams et al., 2012
Vlos map
•Stellar kinematics + stellar pop. synthesis:
σ map
slope=0.9 ±0.15 (core
ruled out at 2σ!!!)
•Gaseous kinematics (slope~0) can be made compatible with stellar
one after taking into account gas motions
•10 more dwarfs in study: a cusp seems to be favored in most of the
cases (Adams+14, in prep), but some cores are also present.
MaNGA/SDSS-IV survey in the next years!
2a. The missing satellite problem
Klypin+ 99
LCDM: Smaller structures collapse earlier and some of them
survive as sub-halos within larger halos. A numerous population
of subhalos is predicted (e.g., ~1000 with Vc>10 km/s in the MW, or
2x106 with M>104 M☉)
However, it seems not to be a problem
predicted
number of sat’s
accounting for
photoionization
Simon & Geha 07
SDSS discovered ~15
ultrafaint dwarfs. LSST and
GAIA may discover more.
Simon & Geha 07
Galaxies in subhalos of
Vc<30 km/s are naturally
suppressed by UV radiation
from reionization!
208
2b. The too-big-to-fail problem
Six Aquarius MW-sized simulations: there are many subhalos with
V>30 km/s that are not seen in the MW: they are too big to not
forming galaxies (Boylan-Kolchin+11,12)
M. Boylan-Kolchin, J. S. Bullock and M. Kaplinghat
Downloaded from http://mnras.oxfordjournals.org/ at UNAM Direcci
gure 3. Rotation curves for all subhaloes with V infall > 30 km s−1 and V max > 10 km s−1 , after excluding MC analogues, in each of the six Aquarius
mulations (top row, from the left-hand to right-hand side: A, B, C; bottom row, from the left-hand to right-hand side: D, E, F). Subhaloes that are at least 2σ
enser than every bright MW dSph are plotted with the solid curves, while the remaining subhaloes are plotted as the dotted curves. Data points with errors
2b. The too-big-to-fail problem
Six Aquarius MW-sized simulations: there are many subhalos with
V>30 km/s that are not seen in the MW: they are too big to not
forming galaxies (Boylan-Kolchin+11,12)
208
M. Boylan-Kolchin, J. S. Bullock and M. Kaplinghat
dark subhalos?
Caveats
•Is the MW + sat’s an
exceptional system?
Downloaded from http://mnras.oxfordjournals.org/ at UNAM Direcci
(Purcell & Zentner12)
•Is the MW halo mass
<1012 M☉? (Wang+12)
gure 3. Rotation curves for all subhaloes with V infall > 30 km s−1 and V max > 10 km s−1 , after excluding MC analogues, in each of the six Aquarius
mulations (top row, from the left-hand to right-hand side: A, B, C; bottom row, from the left-hand to right-hand side: D, E, F). Subhaloes that are at least 2σ
enser than every bright MW dSph are plotted with the solid curves, while the remaining subhaloes are plotted as the dotted curves. Data points with errors
Can we have statistics
for many galaxies?
Rodriguez-Puebla+2012, 2013a, ApJ: Semi-empirical model of galaxy(sub)halo connection and halo occupation -->
A mock catalog of 2 106 central galaxies, each one with their satellites (by
construction it reproduces the observed cen/sat GSMFs & 2-point corr. functions).
1σ
It’s an extension
of observations
logMh= 12.31+ 0.27
halo mass distribution
Probability
Rodriguez-Puebla+2013b, ApJ 773,172
Select those centrals with MW mass (logMs=10.74+0.1): ~41000 gal’s
satellite masses:
ms>2.5×107M☉
The inner dynamics of the satellites
Rodriguez-Puebla+2013b, ApJ 773,172
We have 41K ‘quasi-observed’ MW systems. By construction, there is
not a subhalo-satellite abundance problem for the LCDM, BUT...
Halo/subhalo max. circular velocity
vs central/satellite stellar mass
The inner dynamics of the satellites
Rodriguez-Puebla+2013b, ApJ 773,172
We have 41K ‘quasi-observed’ MW systems. By construction, there is
not a subhalo-satellite abundance problem for the LCDM, BUT...
Halo/subhalo max. circular velocity
vs central/satellite stellar mass
x Observations: isolated and
satellite dwarf galaxies (Geha
+2006)
The inner dynamics of the satellites
Rodriguez-Puebla+2013b, ApJ 773,172
We have 41K ‘quasi-observed’ MW systems. By construction, there is
not a subhalo-satellite abundance problem for the LCDM, BUT...
Halo/subhalo max. circular velocity
vs central/satellite stellar mass
x Observations: isolated and
satellite dwarf galaxies (Geha
+2006)
symbols = MW (massive) satellites
The inner dynamics of the satellites
Rodriguez-Puebla+2013b, ApJ 773,172
We have 41K ‘quasi-observed’ MW systems. By construction, there is
not a subhalo-satellite abundance problem for the LCDM, BUT...
agree
problem?
Halo/subhalo max. circular velocity
vs central/satellite stellar mass
x Observations: isolated and
satellite dwarf galaxies (Geha
+2006)
symbols = MW (massive) satellites
The subhalos of dwarf sat’s (Ms<108) are too concentrated w.r.t.
(few) observations. Does remain this issue for isolated halos?
being satellite could make the difference
3. The Downsizing problem
From empirical inferences: the smaller the galaxies, the latter they
assemble on average their stellar masses, opposite to the CDM
halo trend (Firmani & Avila-Reese10; Behroozi+13; Leitner 12; Moster+13)
Halo masses are rescaled
Halo and stellar mass growth histories
(See Gonzalez-Samaniego’s poster)
3. The Downsizing problem
From empirical inferences: the smaller the galaxies, the latter they
assemble on average their stellar masses, opposite to the CDM
halo trend (Firmani & Avila-Reese10; Behroozi+13; Leitner 12; Moster+13)
Halo masses are rescaled
Halo and stellar mass growth histories
----- Small halos assembled half
their masses earlier on average
than large halos (hierarchy)
____ The corresponding small
galaxies assembled their Ms later
than large galaxies (downsizing)
(See Gonzalez-Samaniego’s poster)
High-resolution hydro/Nbody simulations of lowmass galaxies:
Rotating disks with
nearly flat rotation
curves, realistic
structures and ISM
properties!
H-ART code: Colin+10; Avila-Reese+11;
Gonzalez-Samaniego+14
GADGET code: de Rossi, Avila-Reese+13
Follow the observed
scaling relations
stellar density
Strong SN-driven
outflows.
IGM: infalling and
outflowing gas
SN-feedback and SF
processes delay
slightly SF as less
massive the halos are
(downsizing)...
temperature
velocity
Evolution of the stellar-to-halo mass ratio
de Rossi, Avila-Reese+13
MW-sized gal’s: Ms grows as Mh
low-mass gal’s: Ms growth is delayed
w.r.t. to the early halo growth
Evolution of the stellar-to-halo mass ratio
de Rossi, Avila-Reese+13
MW-sized gal’s: Ms grows as Mh
low-mass gal’s: Ms growth is delayed
w.r.t. to the early halo growth
Empirical inferences
...BUT not enough as observations suggest; Ms growth does
not detach significantly from Mh growth for low mass galaxies!
but see Brooks+12; Munch+13 (extreme ejective SN feedback)
Contents
• LCDM, the simplest cosmological scenario
• What are the LCDM controversies on small
scales?
• Is baryon physics behind the controversies?
• Relax assumptions: dark matter alternatives
• Key challenges
• Concluding remarks
To have in mind
The strongly dissipative baryonic physics affects the
dynamics of dark matter in low-mass halos.
The ‘gastrophysics’ and DM-baryon coupling should
be understood in order to constrain LCDM at small
scales from observations (this applies to any other
alternative scenario!)
Current controversies at small scales: cuspy, too
concentrated (sub)halos, missing satellites and
downsizing could be all related to baryon effects in
low mass halos.
Early SN-driven ejective feedback produces shallow cores and it
lowers Vmax in halos <1011 M☉ Oh+11; Governato+12; Teyssier+12
Inner slope at 0.5 kpc vs Ms: observations vs simulations
Do the SFR histories of dwarfs imply as
much SNe in the past as in the simulations?
No: Boylan-Kolchin+12; Garrison-Kimmel+13.
Yes: Amorisco+13 (Fornax & Sculptor sat’s).
For satellite galaxies: early feedback + ram
pressure + tidal heating from the host -->
shallow core (Arraki+12; Zolotov+12).
Contents
• LCDM, the simplest cosmological scenario
• What are the LCDM controversies on small
scales?
• Is baryon physics behind the controversies?
• Relax assumptions: dark matter alternatives
• Key challenges
• Concluding remarks
In case the small-scale issues are real: relax assumptions
LCDM cosmogony: *Gaussian density perturbations, *scale invariant PS
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
Kuhlen et al. / Dark Universe 1 (2012) 50–93
It’s the simplest case M.from
the point of view of cosmic structure formation
53
In case the small-scale issues are real: relax assumptions
LCDM cosmogony: *Gaussian density perturbations, *scale invariant PS
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
Kuhlen et al. / Dark Universe 1 (2012) 50–93
It’s the simplest case M.from
the point of view of cosmic structure formation
cut-off in the PS
small scales are
suppressed
atomic DM
53
WDM candidates: sterile neutrinos (vMSM model, resonantly produced) and
thermal ~keV particles (e.g., gravitinos)
CDM
Colín+ 00, 08; Avila-Reese+01
MW-sized halo
WDM
WDM candidates: sterile neutrinos (vMSM model, resonantly produced) and
thermal ~keV particles (e.g., gravitinos)
CDM
Colín+ 00, 08; Avila-Reese+01
WDM
Abundance of subhalos in a MW halo
1 keV
Colin, Avila-Reese & Valenzuela 00
WDM candidates: sterile neutrinos (vMSM model, resonantly produced) and
thermal ~keV particles (e.g., gravitinos)
CDM
Colín+ 00, 08; Avila-Reese+01
WDM
MW-sized halo
-WDM produces also: less concentrated halos with less angular momentum (Avila-Reese+ 01;
Bode+01), subhalos with smaller Vc (Lowell+11), delays SF in halos close to the cut-off (Colin+)
-But it doesn’t produce soft cores, unless ‘unrealistic’ amounts of thermal relict velocities
are introduced (Colín+ 08;Padirou+ 09; Shao+ 12), as in meta-DM models: neutrinos
decay lately into gravitinos. See also BDM model (De la Macorra 11, PRD)
The HI velocity (or mass) function of galaxies
Velocity function from HI observations vs
The velocity
local environment
simulations
offunction
thein thelocal
universe
23
Halo mass function
1keV
Zavala+ 09
Fig. 13.— VF for the sample of galaxies in the VdR taken taken from the ALFALFA catalogs (square symbols with error bars).
Predictions from our CSs, using fdisk = 0.03, for the observed field of view appear as the dashed (ΛCDM) and dotted (ΛWDM) red
areas, delimited by Poisson error bars. The dashed and dotted lines are predictions using a model where fdisk is a function of halo mass
incorporating the effects of SN feedback. The sensitivity limit of the survey has been included in the results. The vertical solid line marks
the value of Vmax down to which the simulations and observations are both complete.
Menci+13
FIG. 3: Left panel: Halo mass function as a function of halo mass. L
of Eq. (40). The black solid line denotes the result for CDM; the dot
of particle masses (mWDM ≡ mX = {0.25, 0.5 0.75 1.0, 1.25} keV) wi
the same, but with a cut-off mass-scale as given in Eq. (49), and her
between the WDM and CDM mass functions, as a function of halo m
Mfs . Line styles are the same as in left panel.
Challenges for WDM: The good results are for m~1-2 keV, but
• (a) the linear PS of fluctuations at z>2 from the Ly-α forest observations can’t
be reproduced with a cut-off corresponding to these masses (>3.3 keV, Viel+13);
• (b) too few subhalos to host the observed ultrafaint dwarfs (Polisiensky & Ricotti 11)
Claudio Firmani
LCDM cosmogony:
i Brera, via E. Bianchi 46, 23807 Merate (LC),
Italy
*Gaussian
2 May 17; accepted 2002 August 20
and
Claudio
Firmani
perturbations,
*scale
invariant
density
PS
Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Ita
Received 2002 May 17;
accepted
August 20
without a cut-off, *dominate collisionless, non-interacting,
vrms
=02002
particles.
ABSTRACT
ABSTRACT
series of high-resolution
simulations
were performed
study the str
lations were performed to study the Astructure
and substructure
(Spergelnumerical
& Steinhardt
00; Firmani+00,
01,to
MNRAS)
of Milky Way–sized (MW-sized) and cluster-sized halos in a !CDM cosmology w
r-sized halos in a !CDM cosmology
(SI) per unit of particle mass has the form !DM ¼
dark with
matter self-interacting
particles." The cross section
2 g$1 and
¼ !0 ð1=v
where
!
nit of particle mass has the form !DM
(~0 nuclear-scale
cross-section)
vis
velocity in units of 100 km s$1
a constant
in units
of ,cm
100 Þ
100 is the relative
Colín+02:
Cluster
with
".¼Different
0 or 1 werevalues
used. For
small
relative
velocity
in units of 100 km
s$1
for
!0 values of !DM ¼ const (d0.5, " ¼ 0), the cor
z ¼ 0 is typically higher at a given mass for lower values of !0 or, at a given !0, for low
halo
of&!galaxy-sized
density
of the
halos
at
DM ¼ const (d0.5, " ¼ 0), the! core
as
high
as
3.0,
both
clusterand
MW-sized
halos may undergo the gravothermal ca
0
N-body
lower
masses. occurs
For values
of regime because the heat capacity C is positive
wer
values of !0 or, at a given !0, forThe
core expansion
in a stable
maximum
expansion,before
the isothermal
halos
may undergo the gravothermal
catastrophe
z ¼ 0. core is hotter than the periphery and C < 0.
simulations
catastrophe
is triggered.
instability
both the dynamica
because the heat capacity C is positive
in the
center. The
After
the onset can be delayed byvelocity
major mergers and the interaction of cool particles with the hot environment of a hos
hotter
than the periphery and C core
< 0.density
Thenofthe
gravothermal
dependent
SIDM
Self-interaction-->
1:0ð1=v100
clusterand MW-sized halos is similar. Using
!DM ¼ 0:5
t can
be delayed by both the dynamical
of the
halo
by scaling laws of halos inferred from the observa
with the heating
central densities
and
the core
gravothermal
low
surface
brightness
galaxies
and
clusters of galaxies. Regarding the cumulative vm
ticles
with
the
hot
environment
of
a
host
halo.
When
"
¼
1,
the
processes: core
within MW-sized halos, when ð!0 ; "Þ ¼ ð0:1; 0:0Þ, (0.5, 0.0), or (0.5, 1.0) it agrees ro
predictions
agree
is similar. Using !DM ¼ 0:5 1:0ð1=v
$1, while at v
$1 the functions are
100 Þ, our
e30
km
s
¼
20
km
s
(luminous
satellites)
for
v
max
max
expansion (but
laws of halos inferred from the observations
dwarf
and For ð!0 ; "Þ ¼ ð1:0; 1:0Þ, this function lies a
higher, similarof
to both
the CDM
predictions.
also coreRegarding
collapse) the cumulative
CDM
function.
The structure
and number of subhalos are affected by the scatterin
function
of subhalos
f galaxies.
vmax
halo rather than by those of the subhalos. The halos with SI have more specific angula
0:0Þ, (0.5, 0.0), or (0.5, 1.0) it agrees
roughly
with
observations
mass
shell
and
are
rounder
than their CDM counterparts. However, the angular
$1
ile at vmax ¼ 20 km s the functions
aretoalready
a factor
of 5–8 that the introduction of SI particles with !DM /
regard
CDM is small.
We conclude
core problem
of the CDM cosmogony, at the same time keeping
a subhalo pop
lies above
the corresponding
ð!0 ; "Þ ¼ ð1:0; 1:0Þ, this function cuspy
velocity
the CDMproperties
halos.
f subhalos are affected by the scattering
of the host
dependent SIDM
Subject headings: cosmology: theory — dark matter — galaxies: formation — gala
e halos with SI have more specific angular
momentum
atn-body
a given
methods:
simulations
M counterparts. However, the angular momentum excess with
may remedy the
e introduction of SI particles with !DM / 1=v
1. 100
INTRODUCTION
Recently, a plethora of al
the predictions of the CDM
at the same time keeping a subhalo population similar to that of
Self-interacting Dark Matter
The current paradigm for cosmic structure formation is
based on the cold dark matter (CDM) cosmological models,
its successes at large scales
Davé et al. 2001 and refer
Claudio Firmani
LCDM cosmogony:
i Brera, via E. Bianchi 46, 23807 Merate (LC),
Italy
*Gaussian
2 May 17; accepted 2002 August 20
and
Claudio
Firmani
perturbations,
*scale
invariant
density
PS
Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Ita
Received 2002 May 17;
accepted
August 20
without a cut-off, *dominate collisionless, non-interacting,
vrms
=02002
particles.
ABSTRACT
ABSTRACT
series of high-resolution
simulations
were performed
study the str
lations were performed to study the Astructure
and substructure
(Spergelnumerical
& Steinhardt
00; Firmani+00,
01,to
MNRAS)
of Milky Way–sized (MW-sized) and cluster-sized halos in a !CDM cosmology w
r-sized halos in a !CDM cosmology
(SI) per unit of particle mass has the form !DM ¼
dark with
matter self-interacting
particles." The cross section
2 g$1 and
¼ !0 ð1=v
where
!
nit of particle mass has the form !DM
(~0 nuclear-scale
cross-section)
vis
velocity in units of 100 km s$1
a constant
in units
of ,cm
100 Þ
100 is the relative
Colín+02: Cluster &
with
".¼Different
0 or 1 werevalues
used. For
small
relative velocity in units of 100 km
s$1
for
!0 values of !DM ¼ const (d0.5, " ¼ 0), the cor
galaxy-sized
halo
0 is typically higher at a given mass for lower values of !0 or, at a given !0, for low
of
!DM ¼ const
(d0.5, " ¼ 0), the!z ¼
core
density
of the
halos
at
as
high
as
3.0,
both
clusterand
MW-sized
halos may undergo the gravothermal ca
0
N-body
simulations
lower
masses. occurs
For values
of regime because the heat capacity C is positive
wer values of !0 or, at a given !0, forThe
core expansion
in a stable
maximum
expansion,before
the isothermal
halos may undergo the gravothermal
catastrophe
z ¼ 0. core is hotter than the periphery and C < 0.
catastrophe
is triggered.
instability
both the dynamica
Soft cores
because
theare
heat capacity C is positive
in the
center. The
After
the onset can be delayed byvelocity
major mergers and the interaction of cool particles with the hot environment of a hos
hotter
the also
periphery and C core
< 0.density
Thenofthe
gravothermal
formedthan
(but cuspy
dependent
SIDM
1:0ð1=v100
clusterand MW-sized halos is similar. Using
!DM ¼ 0:5
t due
canto be
of the
halo
by scaling laws of halos inferred from the observa
with
the heating
central densities
and
the core
coredelayed
collapse!), by
butboth the dynamical
CDM
surface
brightness
clusters of galaxies. Regarding the cumulative vm
ticles
the hotthe
environment of low
a host
halo.
Whengalaxies
" ¼ 1,and
the
whenwith
σ=const,
within MW-sized halos, when ð!0 ; "Þ ¼ ð0:1; 0:0Þ, (0.5, 0.0), or (0.5, 1.0) it agrees ro
¼
0:5
1:0ð1=v
predictions
agree
isrange
similar.
Using
!
100 Þ, our
km s$1, while at vmax ¼ 20 km s$1 the functions are
(luminous
satellites)
for vmax e30
of values forDM
laws of halos inferred from the observations
dwarf
and For ð!0 ; "Þ ¼ ð1:0; 1:0Þ, this function lies a
higher, similarof
to both
the CDM
predictions.
producing viable
CDM
function.
The structure
and number of subhalos are affected by the scatterin
function
of subhalos
f galaxies. Regarding the cumulative
vmax
solutions
small
halo rather than by those of the subhalos. The halos with SI have more specific angula
0:0Þ,
(0.5, is0.0),
or (0.5, 1.0) it agrees
roughly
with
observations
mass
shell
and
are
rounder
than their CDM counterparts. However, the angular
(fine-tuning)
$1
ile at vmax ¼ 20 km s the functions
aretoalready
a factor
of 5–8 that the introduction of SI particles with !DM /
regard
CDM is small.
We conclude
core problem
of the CDM cosmogony, at the same time keeping
a subhalo pop
lies above
the corresponding
ð!0 ; "Þ ¼ ð1:0; 1:0Þ, this function cuspy
velocity
the CDMproperties
halos.
velocity are
(energy)
fAsubhalos
affected by the scattering
of the host
dependent SIDM
Subject headings: cosmology: theory — dark matter — galaxies: formation — gala
e dependent
halos with SI
have more specific angular
momentum
atcore
a given
crosscollapse
!
methods:
n-body
simulations
(negative specific heat)
M section
counterparts.
the angular momentum excess
with
is viableHowever,
for
may remedy the
e galaxy
introduction
of SI particles with !DM / 1=v
1. 100
INTRODUCTION
Recently, a plethora of al
and cluster
the predictions of the CDM
at
the same
time keeping a subhalo population similar to that of
scales:
vdSIDM
Self-interacting Dark Matter
The current paradigm for cosmic structure formation is
based on the cold dark matter (CDM) cosmological models,
its successes at large scales
Davé et al. 2001 and refer
Halo Scaling Laws
observations
Colin+02, ApJ 581, 777
ns
o
i
t
a
v
obser
•
The vdSIDM case is the best one
(soft cores in small halos, almost no
cores in clusters)
Halo Scaling Laws
Colin+02, ApJ
CDM
observations
vdSIDM
Colin+02, ApJ 581, 777
•
The vdSIDM case is the best one
(soft cores in small halos, almost no
cores in clusters)
• The SIDM halos are rounder than
the CDM ones.
vdSIDM
ns
o
i
t
a
v
obser
CDM
•
The subhalo population of the
vdSDM is similar to the CDM one
Renaissance of SIDM:
‘Hidden dark sectors’ in particle
models: velocity-dependent cross
sections (Ackerman+09; Buckely+10; Feng
+10;Tulin 13)
New simulations: Vogelsberger+12; Zavala
+13; Rocha+13; Peter+13
Constraints on collisional DM
DM cores, central spherical halo shapes, near-Maxwellian velocity distributions,
are generic predictions of “astrophysically interesting” SIDM models
Dwarf
MW
Cluster
Core-collapse
effective core
creation in dwarfs
(Zavala+ 13)
vdSIDM models motivated by a
new force in the “dark sector”,
e.g. Yukawa-like, (e.g. Feng+09,
Feng+10, Loeb & Weiner 2011)
NGC 720
Bullet cluster (Randall+2008)
(halo shape)
Peter+2013 (cluster ellipticity)
subhalo evaporation
Gnedin & Ostriker 2001
“hard-sphere”
original idea introduced by
Spergel & Steinhardt 2000
constraints, simple ad hoc velocity-dependent cross-sections of the
form 1/v α were explored (e.g. Colı́n et al. 2002), yielding encouragVobelsberg,
Zavala,
Loeblacked
11 a proper underlying particle physics
ing results
that however
model. It has also been claimed that these velocity-dependent SIDM
models are not able to solve simultaneously the core problem in
DM-dominated systems and the ‘missing satellite problem’ (e.g.
χ
D’Onghia & Burkert 2003).
Loeb & Weiner
(2011) proposed that the possible existence of a
c
Yukawa potential among DM particles can resolve the challenges
facing SIDM with a constant cross-section. The velocity depengalactic
darkmediator
matter haloes
3745
dence ofSelf-interacting
scattering through
the massive
of this dark
force
(similar to a screened Coulomb scattering in a plasma) could make
scattering important at the low velocity dispersion of dwarf galaxies
but unimportant at the much higher velocities encountered in galaxy
clusters. The possibility of exothermic reactions could in addition
introduce a special velocity scale around which the influence of the
DM interaction peaks. The existence of dark forces was studied
earlier as a solution to cosmic ray anomalies through enhanced dark
matter annihilation (Arkani-Hamed et al. 2009).
2.1 Velocity-dependent SIDM models
We use a simplified particle physics model where the self-scattering
between DM particles of mass m is set by an attractive Yukawa
potential with coupling strength α mediated by a new gauge boson
of mass mφ (either scalar or vector) in the dark sector. We refer
the reader to Feng, Kaplinghat & Yu (2010b), Finkbeiner et3746
al.
(2011) and Loeb & Weiner (2011) for details on the particle physics
model and its available parameter space. If we only consider elastic
interactions, the scattering problem is analogous to the screened
Coulomb scattering in a plasma, which is well fitted by a transfer
cross-section given by
2012 The Authors, MNRAS 423, 3740–3752

Monthly
Notices of the Royal Astronomical Society 2012 RAS
4π 2

−1

β ln(1 + β ),
β < 0.1,


22.7



 8π
σT
2
1.65 −1
3
(1
+
1.5β
)
,
0.1
<
β
<
10
, (1)
β
≈
max
22.7

σT


&2
%


π
1 −1


lnβ + 1 − ln β , β > 103 ,

22.7
2
#
C
Coulomb scattering in a plasma, which is
cross-section given by

4π 2


β ln(1 + β −1 ),


22.7



 8π
σT
β 2 (1 + 1.5β 1.65 )−1 ,
max ≈  22.7
σT


&2
%


π
1 −1


lnβ + 1 − ln β ,

22.7
2
well fitted by a transfer
β < 0.1,
0.1 < β < 103 ,
(1)
β > 103 ,
2
where β = πvmax
/v 2 = 2αc mφ /(mχ v 2 ) and σTmax = 22.7/m2φ ,
and v is the relative velocity of the DM particles. Here v max is the
velocity at which (σ T v) peaks at a transfer cross-section equal to
σTmax .
The value of σTmax /mχ is constrained by different astrophysical
measurements, the most stringent being (i) the observed ellipsoidal
shape of haloes as implied by X-ray data of galaxy clusters and
ellipticals (Miralda-Escudé 2002; Feng, Kaplinghat & Yu 2010a);
(ii) avoidance of the gravothermal catastrophe leading to inner density profiles even steeper than those predicted in CDM (Firmani
M. Vogelsberger, J. Zavala and A. Loeb
#
C
2
where β = πvmax
/v 2 = 2αc mφ /(mχ v 2 ) and σTmax = 22.7/m2φ ,
and v is the relative velocity of the DM particles. Here v max is the
velocity at which (σ T v) peaks at a transfer cross-section equal to
σTmax .
The value of σTmax /mχ is constrained by different astrophysical
Figure 4. Left-hand panel: density profile of the Aquarius Aq-A main halo at resolutio
measurements, the most stringent being (i) the observed ellipsoidal
reference points we consider (see Table 1) as shown in the legend. Right-hand panel: mean
shape of haloes as implied by X-ray data of galaxy clusters softening
and length (2.8 times Plummer equivalent) of each resolution is marked by a vertica
We achieve good convergence in all our runs, with the inner profiles changing significant
ellipticals (Miralda-Escudé 2002; Feng, Kaplinghat & Yu 2010a);
the largest difference having a large core due to the constant scattering cross-section. We
and is shown here just as reference.
(ii) avoidance of the gravothermal catastrophe leading to inner denFigure 3. Density projections of the Aq-A halo for the different DM models of Table 1 (RefP0-3). The projection cube has a side length of 270 kpc. Clearly,
sity RefP1
profiles
even
than
predicted
in CDM
(Firmani
the disfavoured
model with a large
constantsteeper
cross-section produces
a very those
different density
distribution with a spherical
core in the centre,
contrary to
simulation. In contrast, RefP2 and RefP3 result in a main halo
The subhalos are less
concentrated than in LCDM
the elliptical and cuspy CDM halo. Furthermore, substructures are less dense and more spherical in this simulation. The vdSIDM models RefP2 and RefP3 on
the other hand can hardly be distinguished from the CDM case (RefP0).
whose density profile follows very closely the one from the CDM
prediction of RefP0 down to 1 kpc from the centre. At smaller radii,
LCDM cosmogony: *Gaussian density perturbations, *scale invariant PS
without a cut-off, *dominate collisionless, non-interacting, vrms=0 particles.
8
AVILA-REESE ET AL.
Avila-Reese, Colín, Piccinelli 03, ApJ
Positive and negative
skewed statistics
Positive: more cuspy and
concentrated halos, less
ang, momentum
(a)
(c)
(e)
(b)
(d)
(f)
Negative: shallower and
less concentrated halos,
more ang. momentum
Fig. 3.— Density profiles of halos re-simulated with high resolution. Upper and lower panels are for the skew-positive and
skew-negative statistics. The first and second columns are for the high- and low-concentration galaxy-sized halos, respectively,
while the third column is for the only simulated cluster-sized halo. Solid lines are for the non-Gaussian halos and dashed lines are
for the corresponding Gaussian counterpart (simulated with the same random see). Dotted lines are the best NFW fitting to the
Contents
• LCDM, the simplest cosmological scenario
• What are the LCDM controversies on small
scales?
• Is baryon physics behind the controversies?
• Relax assumptions: dark matter alternatives
• Key challenges
• Concluding remarks
LCDM makes a strong prediction: large population
of dark subhalos around galaxies (absent in WDM).
How to detect them?
-Flux anomalies (Dalal & Kochanek 02; Metcalf & Amara 12) or
astrometric deviations (Vegetti+12) in QSO gravitational lenses
produced by subhalos.
Current detections (e.g., CLASS survey)
are marginal and, if any, imply more
substructure than CDM! Other sources
(e.g., globular clusters, distant galaxies)
should be taken into account (Xu+10).
- Perturbations of the tidal streams
in the MW by the subhalos (Carlberg
& Grillmaiur 13)... preliminary evidence. GAIA.
-Ultra-faint galaxies in the MW, a
proof of small-scale structure
GAIA: Potential to find
them in the multi-space of
stellar coordinates, velocities
and metallicities
-Triaxiality of the galaxy dark halos
from stellar and gaseous kinematics
(Integral Field Spectroscopy -MaNGA)
-Power spectrum of gas clouds in H21
cm line at z>10 (proto-structures at all
scales)
known
ultra-faint
galaxies
To take home
A better understanding of the gastrophysics and the
baryon-dark matter interaction is necessary to
acquire before claiming for issues at small scales
Key tests for LCDM: discovery of the predicted
abundant substructure and the halo triaxiality
The dark sector is likely much more complex than
our simple models, with its own (self)interactions and
particle zoo.
SIDM is promising.