The bar/bulge region in our Galaxy

The bar/bulge region in our Galaxy
Lia Athanassoula
LAM Marseille
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Why do we want to study the the bar/bulge region?
The bar is a main driver for the secular evolution of disc galaxies
Pushes mass from the bar region to the center where it creates a
CMC (central mass concentration)
Drives the angular momentum exchange within a disc galaxy:
Angular momentum is emitted by near-resonant material in the
bar region and is absorbed by near-resonant material mainly in
the halo but also in the outer disc
The strength of the bar correlates well with the amount of
angular momentum exchanged
(Athanassoula 2003)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Bars and Boxy/Peanut/X bulges
Bars
rotate!
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Why do we want to study the the bar/bulge region?
The bar is a main driver for the secular evolution of disc galaxies
Pushes mass from the bar region to the center where it creates a
CMC (central mass concentration)
Drives the angular momentum exchange within a disc galaxy:
Angular momentum is emitted by near-resonant material in the
bar region and is absorbed by near-resonant material mainly in
the halo but also in the outer disc
The strength of the bar correlates well with the amount of
angular momentum exchanged
(Athanassoula 2003)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Angular momentum lost by bar: How?
- orbits become
thinner
Thinner
- bar traps stars
which where on
near-circular orbits
around it, into its
outer parts
- bar rotates
slower
The bar/bulge region in our Galaxy
Longer and
more massive
So when bars lose angular momentum they slow down
and they become stronger (longer, more massive and
thinner)
NAOC
October 2012
Lia Athanassoula
No bar
|
bar
|
growth
|
bar evolution
Secular evolution
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Angular momentum transfer
in disc
in spheroid
Angular moment absorbed by the spheroid (halo + bulge): The spheroid
material acquires some rotation. The bulge and inner halo more so.
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Haloes should be adequately modelled
Athanassoula 2002
Rigid halo
Live halo
Halo can not receive
angular momentum
Halo can receive
angular momentum
No bar develops
Strong bar develops
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Emitters and absorbers
Example from a strong bar model
Athanassoula 2003
Confirmed by Martinez-Valpuesta+ 2006, Ceverino+ 2007, Villa-Vargas+ 2009,
Saha+ 2011 etc (different models, codes etc)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Why do we want to study the the bar/bulge region?
The strength of the bar correlates well with the amount of
angular momentum exchanged
(Athanassoula 2003)
We could be witnessing the interaction between the three
different types of bulges: classical, boxy/peanut and discy.
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Orbital structure in bars
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Peanuts should be SHORTER than bars
edge-on
face-on
Athanassoula 05
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Peanuts are shorter than bars in simulations
Thin and thick
parts of bars
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Observed peanuts are shorter than bars
A thin component in boxy/peanut bulges
(observations)
Quantitative estimates :
Simulations :
Athanassoula and Misiriotis 2002
Athanassoula 05
Athanassoula and Beaton 2006
Orbital structure :
Pfenniger 1984
Patsis, Skokos and Athanassoula 2002
Lutticke, Dettmar and Pohlen, 2000
Thin and thick parts of bars
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
For a full movie see
http://lam.oamp.fr/research/dynamique-des-galaxies/
scientific-results/milky-way/bar-bulge/how-many-bars-in-mw
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
A tale of two bars
Benjamin
Our Galaxy is barred
COBE/DIRBE bar (Binney et al 97)
Signal for a second bar:
The Long bar
Hamersley et al 2001
Benjamin et al 2005
Lopez-Corredoira et al 2005, 2007
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
A single bar in the Galaxy
So how are the COBE/DIRBE bar and the Long bar related?
Clue: Long bar is vertically very thin, COBE/DIRBE bar is very thick.
Athanassoula (2006): There is a single bar of which the COBE/DIRBE bar
is the boxy/peanut part and the Long bar is the thin outer parts. Tested by
Cabrera-Lavers et al (2007).
Cabrera-Lavers 2007
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
But:
The difference in position angles? (20 and 40 degrees)
Arguments summarised in Romero-Gomez et al (2011).
See also Martinez-Valpuesta and Gerhard (2011). Good agreement
Zasowski, Benjamin and Majewski (2011)
The long bar is at 25 – 35 degrees
The bar/bulge region in our Galaxy
NAOC
Face-on view of the bar: The B/P part
is thicker than the outer part. This can
contribute to the angle difference
between the Long 'bar' and the
COBE/DIRBE 'bar'
October 2012
Lia Athanassoula
For a full movie see
http://lam.oamp.fr/research/dynamique-des-galaxies/
scientific-results/milky-way/bar-bulge/how-many-bars-in-mw
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
A leading extension in
the ring: This may be
the reason that we see
the long bar at a larger
angle than the COBE/
DIRBE bar (or may
contribute substantially
to it ). It is sufficient, but
Not necessary
Feature found in:
Athanassoula and
Misiriotis 02
NGC 1808
The bar/bulge region in our Galaxy
NAOC
October 2012
Use for the MW:
Romero-Gomez et al 2011
Lia Athanassoula
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
X shapes
NGC 4710 unsharp masked
Aronica, Athanassoula, Bureau, Bosma
et al (2003)
Bureau, Aronica, Athanassoula,
Dettmar, Bosma, Freeman 2006
N-body simulation
Athanassoula (2005)
3-D periodic orbit calculation
Patsis, Skokos and Athanassoula (2002
)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Unsharp masking simulations
from different viewing angles
Athanassoula 2005
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
McWilliam & Zoccali 2010
Nataf et al 2010
etc
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
ARGOS spectroscopic survey
AAOmega fibre spectrograph on the AAT
Spectral region 8400 – 8800
Resolution about 11 000
28 fields, about 1000 stars per field
Stars selected from the 2MASS Survey
11.5 < K < 14
Errors in J, K < 0.06
Include stars in red rectangle
Includes bulge giants + excludes foreground disc dwarfs
Radial velocities (1.2 km/sec)
[Fe/H], [alpha/Fe]
Ness et al 2012a,b,c,d
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
ARGOS spectroscopical survey (Ness et al 2012a)
Simulations:
3 slices in z
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Including gas and non-spherical haloes
Include gas with sub-grid physics (SF, feedback, cooling .. )
(initial gas fraction: 0%, 20%, 50%, 75%, 100%)
200K to 4M SPH particles
Athanassoula, Machado and Rodionov 12 (=AMR12)
50 oc resolution
Non-spherical haloes
Compare 3 haloes;
Spherical
mildly triaxial (initially 0.6 and 0.8)
Strongly triaxial (initially 04 and 0.6)
No initial bulk rotation
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Halo axial ratio in
the equatorial plane
AMR12
For the 0% gas :
Good agreement with
previous results
Athanassoula 05, 07
Colin et al 2006
Berentzen et al 06, 07
Machado &
Athanassoula 2010
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
AMR12
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
AMR12
t > 6 Gyrs
The bar/bulge region in our Galaxy
t < 6 Gyrs
NAOC
October 2012
Gas
Lia Athanassoula
Effect of halo triaxiality:
Initial stages:
The bar grows faster in a triaxial halo
Secular stages:
The bar grows slower.
Sufficient triaxiality can stop the secular evolution
AMR12
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Angular momentum absorbed by the halo manifests itself as:
1) Rotation of the spheroid (= halo and bulge) material that is nearest to the disc
2) If halo is triaxial, some bulk rotation (but little)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Are bars long-lived or short-lived?
Orbits and gas-less simulations
CMC of mass = 0.1Mdisc, if compact
Much more if extended
So long-lived
Simulations with a gas component:
Short lived:
Bournaud & Combes 2002, 3 – 4 episodes of bar formation
Criticism: Numerical effects, poor time integration (Shen & Sellwood 2004)
Bournaud, Combes & Semelin 2005: Lifetime of 1-2 Gyr
Criticism: Rigid halo, i.e. no resonances (Athanassoula 2002, Berentzen et al 2007)
Long lived:
Berentzen et al 2007; Villa-Vargas et al 2010;
Criticism: Not enough SPH particles (40 000 SPH particles)
Our simulations (AMR):
Live halo --- Resolution sufficient to describe properly the resonances
Large number of gas particles --- up to 4 millions !!
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Our simulations (AMR):
Live halo --- Resolution sufficient to describe properly the resonances
Bar srength
Large number of gas particles --- up to 4 millions !!
Time
Bars are not destroyed, even in simulations with 100% gas!!
They are therefore long-lived
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Bars in cosmological simulations
Scannapieco & Athanassoula 2012
Athanassoula & Scannapieco 2012
2 Milky-Way-mass disc galaxies
IC: Aquarius project (DM only) Aq-C-5, Aq-G-5
Zoom simulations include baryons
Gadget3 (Springel et al 08)
SF and feedback (Scannapieco et al 05, 06)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Rotation curves
Athanassoula & Scannapieco 2012
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
Bar strength
Athanassoula & Scannapieco 2012
(more massive)
The bar is long-lived !!!
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
As a result of this angular momentum exchange:
Bars become stronger, longer and somewhat thinner.
They also rotate slower.
Haloes acquire some rotation and become triaxial (inner parts).
Discs become more extended
The bar pushes material inwards and forms the discy bulge (or pseudo
bulge), but it also becomes vertically thicker (boxy/peanut bulge)
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula
The end
The bar/bulge region in our Galaxy
NAOC
October 2012
Lia Athanassoula