The formation of realistic disk galaxies in

1
The formation of realistic disk galaxies in LCDM
simulations
Lucio Mayer
With Fabio Governato (UW), Beth Willman (CfA
Harvard), Chris Brook (UW), Alyson Brooks (UW),
Greg Stinson (McMaster), Tom Quinn (UW), James
Wadsley (McMaster), Tobias Kaufmann (UC Irvine),
Simone Callegari (PhD student, U. of Zurich),
Geppina Coppola (PhD student, U. of Zurich)
2
Angular Momentum and Concentration Problem
Disks rotate too fast at a given luminosity -gt
disks too concentrated hence Vrot
(GM/Rdisk)1/2 too high
Disks are too small at a given rotation speed
Courteau 1997
N (gas, dm) 104 particles per galaxy
Navarro Steinmetz 2000
Both in observations and simulations
Jdisk2RdVrot, where Rd is computed by fitting
an exponential profile to the stellar surface
density
3
Simulationsof Disk Galaxy Formation have
improveddramatically over the past few years.
How?

• Numerics/Mass and spatial resolution (N 10k to
  3-5 million grav. softening from 1 kpc to 100
  pc), e.g. Okamoto et al. 2003 Sommer-Larsen et
  al. 2003 Governato, Mayer et al. 2004 Kaufmann,
  Mayer et al. 2007Mayer, Governato Kaufmann
  2008
• for a review)
• Description of ISM thermodynamics/Star formation

Thacker Couchman 2001 Governato, Mayer et al.
2004 Abadi et al. 2003 Sommer-Larsen et al.
2004 Brook et al. 2005 Robertson et al. 2005
Governato, Willman, Mayer et al. 2007 Robertson
Kravtsov 2007 Ceverino Klypin 2008
4
Spurious angular momentum lossMW-sized model
(Vcirc 160 km/s, c10, fb0.1,l0.045)
Kaufmann, Mayer et al. 2007
Ngas5x10
5
Ngas5 x 105
Ngas8 x 104
Ngas104
Ngas
4
Ngas10
5
Ndm10
35 kpc
35 kpc
35 kpc

• Conservation of J improves with increasing Ngas.
• Convergence not reached even with 106 gas
  particles but loss of angular momentum down to
  10-20
• Artificial loss of J in SPH simulations due to
• (1)Artificial viscosity torques (2) spurious
  hydro torques between cold disk and surrounding
  hot phase (3) spurious gravitational torques
  between cold gas and hot halo

Gas particles in a sphere of initial radius
cooling radius 80 kpc are followed. These
particles end up in the disk , i.e. they trace
the angular momentum evolution of the disk as it
grows inside-out
5
High resolution galaxy formation
(Governato, Willman, Mayer et al. 2007 Mayer,
Governato Kaufmann 2008).
Volume renormalization technique 300 pc
spatial resolution in a 100Mpc box (DM GAS)
Large scale tidal torques preserved, crucial to
get right angular momentum of halos Halo
s of different masses selected (1011- 3 x 1012
Mo), mostly with no major mergers for z lt 1.5
6
Star Formation and Feedback (Stinson et al. 2006
for details)

• Star particles formed from cold, dense gas
  (density threshold)
• Kroupa IMF
• Mass/metals loss from stellar winds included.
• Uniform, time-dependent cosmic UV bg from Haardt
  Madau
• Supernove explosions, O and Fe yields from SN I
  II

• Supernovae Feedback with Blastwave model
• Based on multiphase ISM model of McKee Ostriker
  (1977)
• Damp fraction of explosion energy to thermal
  energy of gas
• Rad. cooling stopped during adiabatic expansion
  phase of
• supernova blast wave (Sedov-Taylor phase) 2 x
  107 years
• Result no powerful outflows (no gas mass loss)
  but star formation quenched because at all times
  some gas is in warm, diffuse gas phase (T gt 30000
  K)

7
Effect of increasing resolution on the size of
disks MW-sized galaxy (halo has 1012
Mo, l 0.05)
NDMGasstars, frames 60 kpc
Images with SUNRISE (Patrik Jonsson)

Rd30 smaller
8
Circular velocity profiles vs. resolution revisit
ing the mass concentration problem
Low-res 3.5 x 104 dark matter and gas/star
particles Average res 3.5 x 105 dark matter
and gas/star particles Hi-res 2 x 106 dark
matter and gas/star particles
Mayer, Governato Kaufmann, 2008
At high resolution rotation curve begins to
resemble that of an early-type spiral galaxy
(e.g. M31)
9


10
The Velocity- Size Relation
Courteau 07
High Res galaxies (Np gt 106)
V W20/2 Mag Sunrise
11
But Feedback crucial to regulate star
formation/cold gas
Effect of SN feedback on SFH of a 1011 Solar
Masses Galaxy
Without blastwave feedback star
formation history follows merging history. If
blastwave feedback is on, star formation peaks
at zlt 1 AFTER Last Major
Merger. Early mergers inefficient and gas
rich SF in bulges suppressed.
Last Major Merger
Feedback OFF
Feedback ON
SFH includes all progenitors at any given time
12
Obs. plot byMac Arthur Courteau and Bell 2004
A G E Of S T E L L A R P O P
Runs with SN feedback (filled symbols)
consistent with the observed Vrot vs Age
trend. Star Formation delayed/suppressed in
small progenitors.

13
Effect of increasing resolution and/orfeedback
on relativecontribution tototal stellar
mass-Stellar Haloless massive.-- Disk more
Massive.-- Bulge less massive

• Components of simulated MW-sized galaxies.
  Kinematic decomposition.

Halo
Bulge
MR 3.5 x 105 dark matter and gas/star
particles HR 2 x 106 dark matter and gas/star
particles
Disk
14
Gas Rich Disks are resilient and do not
need quiet initial conditions to form Last
Major Merger at z1 7 accretion episodes, many
are 110 mergers

Gasgreen Blue Red Stars Z50 to z0.
Frame size 200 kpc comoving
15

• Large HI disks already in place at z gt 2
• Disk formation Is not simply inside-out
• mergers do play a major role in disk growth
• Gas-rich major mergers can build large disk
  galaxies -
• first evidence in a cosmological simulation
• Agreement with binary mergers by Robertson et al.
  2006 but does not require gas fraction in disks
  as high as 50 before final merger gas fraction
  in disks here is 10-20 but 3x times more gas is
  in the halo cold flows within 20 kpc from the
  disk
• -? (high) Orbital angular momentum converted into
  angular momentum of fresh infalling gas after
  merger

16
Galactic HVCs, HI clouds and extraplanar gas
Evidence for clumpy gas accretion at z0?
Grossi et al. 2008 (ALFALFA survey))
Thilker et al. 2003
M33
M31

• Mgas 106 - 109 Mo
• nHI 1018-1021 cm-2
• -? future revolution with
• SKA, survey with x1000
• galaxies, probe down to
• 104 Mo

Fraternali et al. 2004
17
Structure of the ISM/IGM at z 0.5 Lots of
structure in cold (lt104 K) gas as tracer of
galaxy formation Gas
clouds/structure well resolved only down to 106
solar masses, resolution 0.3Kpc
Hot Halo (Blue)

Ram Pressure Stripping
Gas Rich Satellites
High Velocity Clouds
Cold Gas in Disks
250Kpc across resolution 0.3Kpc
18
Successes and Open Issues

• Disk galaxies are easier to form and more
  resilient than we
• originally thought in LCDM models gas-rich
  mergers are favourable

• Higher resolution and physically motivated star
  formation/feedback
• are necessary ingredients to form more realistic
  galaxies
• Cosmological disk formation simulations have now
  enough resolution
• and reliability to allow studying the various
  gas phases (clouds eventually
• formed by thermal instability, cold
  cosmological flows -- see
• Avishai Dekels talk and Chris Brooks talk --
• hot corona, tidal debris gas etc..)

But how do we form bulgeless galaxies? 30 of
disk galaxies at z0 and and even more common
at z 0.5-1 see COSMOS survey (Sargent et
al. 2007) Major problem because mergers natural
in LCDM and mergers always produce/increase a
central bulge if baryons cold(stars, cold gas)