BAR FORMATION IN COSMOLOGICAL FRAMEWORK

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BAR FORMATION IN COSMOLOGICAL FRAMEWORK

• Anna Curir
• Paola Mazzei
• Giuseppe Murante
• INAF - Osservatorio Astronomico di Torino
• INAF - Osservatorio Astronomico di Padova

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Bar instability in disksinside Dark Matter
haloes

• Passive halo live halo
• Non rotating halo spinning halo
  (Curir Mazzei 99,01)
• Spherical halo triaxial halo
• Relaxed halo unrelaxed halo (
  Curir Mazzei 99, 01)

HOW THE DISK IS REACTING TO SUCH HALOS MODELS?
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Athanassoula (2001)

• The halo can stimulate the bar formation,
  contrary to the common belief that it will
  always quench it
• In fact a live halo responds to the bar and a
  considerable fraction of its particles can be in
  resonance with it.
• This effect has been missed by simulations
  treating the halo as a rigid component

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Bar formation inside Cosmological DM halos

• We adopt fully cosmological DM haloes, inside a
  cosmological scenario, to embed a stellar disk.
• In this way we can investigate the role of the
  infall, the influence of the Dark Matter
  structure (triaxiality, substructure..), of the
  cosmological expansion ..
• We get the disk evolution as function of the
  redshift

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• ?CDM cosmology ,i.e.O?0.7, Om0.3, H070
  km/s/Mpc.
• The box size is 25/h Mpc.
• The halo has been selected in a low-resolution
  run (1283 particles),
• it doesn't suffer major mergers since z5 and
  lives in a low-density
• environment. The halo identification is done
  with a friends of friends
• algorithm at z0.
• We re-simulate the halo at 8 times higher
  (linear) resolution,
• following the whole simulation box with a
  multi-mass technique to
• account for the large-scale tidal forces.
• The disk is embedded in the halo at a chosen
  redshift .
• The total number of dark matter particles in the
  high resolution
• region is 1216512, which corresponds to a DM
  mass resolution of
• 1.21 X 106 solar masses
• The high resolution zone is surrounded by three
  shells with lower
• and lower resolution, the lowest one
  including all the remaning
• (not resampled ) particles among the initial
  1283 set

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Accreting history of the halo as function of z
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Disk immersion

• The disk is generated in a plane orthogonal to
  the halo angular momentum.
• Each particle of the disk is initially massless.
  Such a mass increases linearly with tme until the
  final value of disk-to-halo mass ratio.
• After such period the disk is embedded in
  equilibrium with the gravitational potential of
  the halo

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DM halo properties
t (a2- b2)/(a2-c2) triaxiality parameter
(Warren et al. 92)
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Mass resolution and softening

• Halo particle mass 1.21 106 solar masses
• Stellar particle mass same as DM particle in
  the simulation of a disk having the same mass of
  the halo, 0.39 106 and 1.21 105 in the other
  disks having lower masses
• Softening 0.5/h Kpc
• Simulations with higher resolution are now
• in progress

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Our approach allows us to account for the
cosmological field acting on the DM halo and to
accurately follow the evolution of the selected
halo in a self consistent way.

• WE USE THE NUMERICAL TREE-CODE GADGET (V.
  Springel)
• We carried out two sets of simulations embedding
  the galactic disk
• in the halo at the redshifts z2 and z1
  respectively.
• The first choice corresponds to 10.24 Gyr down
  to z0 in our
• chosen cosmology, the second one to 7.71 Gyr.

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The stellar disk

• The spatial distribution of the star particles
  follows an exponential surface density law .
• The disk is consisting of 56000 star particles.
  The scale length is 4 Kpc and the disk is
  truncated at 5 scale lengths.
• Circular velocities are assigned analytically to
  disk particles accounting for the global
  (diskhalo) potential.
• Velocity dispersions are monitored through a
  Toomre-like parameter q.
• We explore two values of q 0.5 and 1.5

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Isolated simulations in a NFW halo

• We performed also isolated simulations of disks
  inside a Navarro Frenk and White halo having the
  same mass radius, number of particles and
  concentration as the cosmological one at z2.
  Such a halo has a cosmological density profile
  but it is isotropic spherical, non rotating
  and in gravitational equilibrium.
• Such simulations give insights into the
  effects of the halo initial dynamical state on
  the growth of bar instability.

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Bar Strength

• As a measure of the bar strength we used the
  value of the axial ratio, S_mb/a a strong bar
  corresponds to an ellipticity epsilon(1-b/a),
  larger than 0.4.
• We also measured the bar strength with the
  parameter Q_t (Combes and Sanders 1981)Q_t (?F
  /??)max/R(?F /?r)

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Cosmological simulationsinitial values
Rdm initial halo-to-disk mass ratio inside the
disk radius
Heavy disk
Intermediate mass disk
Light disk
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Cosmological simulationsfinal bar strength and
length
Heavy disk
Intermediate mass disk
Light disk
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EFFECT of the q PARAMETER on the same disk
q1.5 Z0
q0.5 Z0
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Ellipticity as function of the semimajor axis and
at various z
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Bar strength evolution
Evolution of the bar strength after z1 for
simulations 2 (full line), 4 (dotted line)
and 6 (dashed line)
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Less massive disk at z0
Intermediate mass disk at z0
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The light disk immersed in the NFW halo
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The two bars formed in Cosmology and inside the
NFW halo
Cosmological halo
NFW halo
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Evolution of the density profile in the NFW halo
and in the cosmological one From top to bottom
z0, z0.5, z1 z1.5
Z0
Z0 Z0.5 Z1 Z0.5
Z0 Z0.5 Z1 Z1.5
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Stellar - gaseous disks

• We are currently performing simulation of the
  same systems with different percentages of the
  disks mass converted in gaseous mass.
• The gaseous component is consisting of 56000
  gas particles initially distributed with the same
  density distribution of the stars

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Our intermediate mass case in the same halo,
with a gas fraction of 0.2
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Final snapshots of simulations of self
gravitating disks with two different gas
percentages
Stars
Gas
0.1 gas mass
0.2 gas mass
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Final snapshots of simulations of light disks
with different gas percentages
0.1 gas
0.2 gas
0.4 gas
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• The final bar strength of the intermediate mass
  disk is decreased of 50 if we include a
  fraction of gas equal to 0.1 of the stellar disk
• In the same disk with 0.2 disk mass of gas the
  bar disappears at z0
• For the light disk case the final bar strength is
  decreased of 20 with 0.1 disk mass of gas and of
  30 for a fraction 0.2

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Conclusions I

• Stellar disks of different masses and q
  embedded in the same halo and evolving in
  cosmological scenario, develop long living bars
  lasting up to redshift 0.
• The strength of the bar at z0 is weakly
  depending on the q parameter colder disks show
  stronger and longer bars.

The embedding red shift does not have a major
impact on the bar strength, but it has an
impact on the bar length
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Conclusions II

• The same disk immersed in a NFW halo having the
  same mass, radius and concentration of the
• cosmological one develop a stronger bar in the
  more massive case, but the bar does not develop
• in the lighter disk case, whereas a short bar
  is evident in the disk embedded in the
  cosmology.
• The full cosmological environment enhances bar
  formation?

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• In our previous papers (Curir and Mazzei 99,01)
• we noticed that inside halos still far from
  relaxation bar formation was more enhanced than
  in halos completely relaxed

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We argue then that an unrelaxed dynamical state
for an isolated halo is the more suitable to
mimic a realistic cosmological halo as far as bar
formation is concerned
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Cosmological halo
Unrelaxed isolated triaxial halo
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THANK YOU