How do Galaxies Get Their Gas

1
The EVLA Vision Galaxies Through Cosmic Time
Dec. 2008
How do Galaxies Get Their Gas?
Duan Kere Institute for Theory and
Computation - Harvard-Smithsonian
CFA Collaborators Neal Katz, David Weinberg,
Romeel Davé, Mark Fardal, Lars Hernquist, T. J.
Cox, Phil Hopkins
2
Active star formation occurs during most of the
Hubble time
Hopkins Beacom 2006
What drives the star formation? Why is SFR
density so high at early times and decreases at
low-z?
3
What drives this star formation evolution?

• Stars form from dense/galactic gas
• Galaxies have very high gas fractions at high
  redshift and extended gas reservoirs -gt can
  provide high SFRs.
• Depletion of this gas can then slow down the star
  formation
• Could explain some of the observed trends
• Lets check if there is enough galactic gas at
  high redshift

4
The state of dense gas

• Damped Ly? Absorbers (DLA) are probing column
  densities typical of the gaseous galactic disks
• Amount of gas in DLA at high-z is less than the
  mass locked in stars at z0.
• A vast majority (80) of stars form after z2
  (e.g. Marchesini et al. 08)
• During the time when most of the stars formed
  dense HI phase stays constant
• Dense gas phase needs to be constantly
  re-supplied.

ProchaskaWolfe 08
5
Is there an evidence for gas supply in our
neighborhood?
6
Milky Way

• MWs SFR 1-3M_sun/yr (Kennicutt 01)
• MWs gas reservoir is 5e9M_sun . Without gas
  supply this is spent in several Gyr.
• Star formation rate in the Solar neighborhood
  relatively constant. Not much change in gas
  density despite large gas depletion (Binney et
  al. 2000)
• Supply of gas is needed

7
HVCs around MW

• Most direct measure of galactic infall
• Many clouds with inwards velocities, net infall
  from known clouds gt 0.25M/yr
• Hard to explain by the galactic fountain
• Low metallicities
• Similar net infall rates from clouds around other
  galaxies

Van Woerden et al. 2004
8

• Conclusion Accretion of gas from the
  intergalactic medium is ongoing at all epochs.

9
The Theory
10
Standard Model

• E.g. White Rees 1978
• Gas falling into a dark matter halo, shocks to
  the virial temperature Tvir at the Rvir, and
  continuously forms quasi-hydrostatic equilibrium
  halo.
• Tvir106(Vcirc / 167 km/s)2 K.
• Hot, virialized gas cools, starting from the
  central parts, it loses its pressure support and
  settles into centrifugally supported disk gt the
  (spiral) galaxy.
• Mergers of disks can later produce spheroids.
• The base for simplified prescriptions used in
  Semi-Analytic models SAMs (e. g. White Frenk
  1991).

Tvir
11
NASA/WMAP
Structure formation Gas heating Gas
cooling Dynamics-merging Star-formation FEEDBACK
Complex and non-linear Simulations needed
Earliest Observation
How do galaxies form and evolve?
Today
NOAO
12
SPH simulations of the galaxy formation

• We adopt ?CDM cosmology.
• Gadget-2 and 3 (Springel 2005) SPH code entropy
  and energy conservation proper treatment of the
  cooling flows
• Cooling (no metals), UV background, a star
  formation prescription
• A simple star formation prescription
• Star formation happens in the two-phase
  sub-resolution model SN pressurize the gas, but
  does not drive outflows
• Star formation timescale is selected to match the
  normalization of the Kennicutt law.
• Typically no galactic winds
• Largest volume 50/h Mpc on a side, with gas
  particle mass 1.e8 M?, but most of the findings
  confirmed with resolution study down to 1.e5 M?
• Lagrangian simulations -gt we have ability to
  follow fluid (particles) in time and space.

13
Global Accretion

• We define galaxies and follow their growth
• Galaxies grow through mergers and accretion of
  gas from the IGM
• Smooth gas accretion dominates global gas supply
• Mergers globally important after z1
• Star formation follows smooth gas accretion,
  owing to short star formation timescales
• It is within factor of 2 from the typical Madau
  plot (e.g. Hopkins Beacom 2006)

Kere et al. 2008
14
Temperature history of accretion

• We utilize the Lagrangian nature of our code
• We follow each accreted gas particle in time and
  determine its maximum temperature - Tmax before
  the accretion event.
• In the standard model one expects Tmax Tvir

Galactic gas
15
Evolution of gas properties of accreted particles

• - Empirical division of 2.e5K, but results are
  robust for 1.5-3e5K
• - The gas that was not heated to high
  temperatures -gt COLD MODE ACCRETION
• - gt Accretion from cold filaments
• - The gas that follows the standard model -gt HOT
  MODE ACCRETION
• - gt Accretion from cooling of the hot halo gas

Kere et al. 2005 Katz, Keres et al. 2002
16
How important are these two accretion modes?

• Cold mode dominates the gas accretion at all
  times.
• Hot mode starting to be globally important only
  at late times

17
The nature of cold and hot modes
18
Low mass halos are not virialized

• Halo gas (excluding galactic gas), within Rvir
• Low mass halos are not virialized
• Transition Mh1011.4 M?
• Approximate description by BirnboimDekel 2003
• Post shock cooling times are shorter than
  compression time at Rvir when Mh1x1011 M?
• More gradual transition in our case because cold
  filaments enhance the density profile

Kere et al 2008
19
Low mass halos
Low mass halos

• High-z well defined filamentary accretion
• Low-z -gt Tvir is lower, filaments are warmer and
  larger than the halo size

20
High-z accretion
Kere et al. 2008

• Strong mass dependence of accretion
• Cold mode infalls on a roughly free fall
  timescale. It follows the growth of DM halos in
  the lower mass halos and still is within a factor
  of 3 in massive halos
• High accretion rates result in high SFRs of high
  redshift galaxies
• Even in massive halos cold mode filaments supply
  galaxies with gas
• Cooling from the hot atmosphere not important.
• Satellites accrete with the same rates as central
  galaxies

21
lt 1kpc resolution, m_p 1.e6 M_sun
Kere et al 2008
22
Zoom to 0.8Rvir

• zgt2 cold mode can supply the central galaxy and
  satellite galaxies efficiency to supply galaxies
  directly declines with time
• z1 cold mode less efficient in massive halos
  lower density contrast -gt easier to disrupt
• It can supply satellites directly but supply to
  the center is limited and often goes trough cold
  clumps

23
Low-z accretion

• Drastic accretion change over time, factor of 30
  from z4 to 0.
• At low mass this roughly follows the drop in the
  dark matter halo accretion
• Some hot mode accretion around the transition
  mass.
• Halos more massive than 1012M? (an order of
  magnitude above transitions) stop cooling hot gas
• At z0, a fraction of massive halos, around MW
  mass is able to cool the hot gas

Red - hot mode Blue - cold mode
Kere et al. 2008
24
Recent future simulations
25
5kpc/h physical
New simulations
Z2.6 110/h pc physical res. M_p1.e5M_sun M_h1.3
e11M_sun Yellow Tvir Blue 1e4K
300/h kpc 2Rvir
1.2/h Mpc cmv.
26
z2.6 25/h kpc (physical)
27
Halo mass at z0, 7x1011M_sun Gas particle
mass 105M_sun
28
Zoom onto a Rvir region, with the lowest
overdensity moving from 200 to 1500
29
Can we detect the smoothly accreting gas with
EVLA?(very preliminary )
30
(No Transcript)
31
High-z

• Filaments have n_HI 1.e18-1.e20, and around
  1.e11Msun halos widths of several tens of kpc.
• Massive halos, around MW size are good targets to
  constrain the accretion models
• This continues to about z1
• Lower mass galaxies are embedded in thick
  filaments
• Need to work on kinematic signatures

32
z1.0 z0.5
z0 400kpc region, 1pixel1kpc, approx 5
at 40Mpc
33
EVLA range z0-0.5

• Hard to detect filaments
• Their physical size and typical temperature
  larger at low redshift 20 to 100 kpc scale. This
  results in low columns.
• Galaxies in MW size halos are surrounded by the
  cold clouds with mixed origin
• No more clear distinction of the cold and hot
  modes. Many clouds form from cooling
  instabilities in warm 1.e5K gas in the outskirts
  of halos. Similar process occurs when filaments
  approach the virial radius.
• Clouds are easily detectable once inside 50kpc.
  A large number of clouds should be visible around
  external galaxies with n_HI gt1.e18 cm-2
• Clouds form in much larger numbers if there are
  galactic winds operating.
• Observations could place some constraints on the
  level of such feedback, cloud formation radii and
  cloud survival.
• Clouds should be common around MW size galaxies
  that are central in halos. Less common in lower
  mass halos.
• More work is needed for more massive objects.

34
Where should an idealized HI telescope look?

• Higher redshift -gt more coherent structure
• Higher mass galaxies -gt higher density contrast
• Low mass galaxies, relatively more HI gas
  compared to the galaxy size
• no clouds far from the galaxy, harder to detect
  extended halo
• z1 is already an interesting regime.

35
Summary

• Low mass halos do not undergo classic
  virialization
• In more massive halos gas is heated to Tvir
• The transition into the hot halos is gradual
  because of the filaments that enhance the density
  structure in a halo
• At high redshift (z gt 1.5) smooth gas accretion
  is completely dominated by the cold mode
  accretion, even in massive halos
• Cooling from the hot atmosphere is inefficient at
  all times
• At low redshift hot mode is important in a
  fraction of objects but the most massive halos
  accrete very little from the hot atmosphere
• Accretion of cold clouds with mixed hot and cold
  mode origin dominates the late time accretion in
  MW size halos, but more work is needed to
  understand the physics of this process.
• EVLA could probe the last epoch where coherent
  structures feed galaxies and should be great for
  studies of cold (HVC like) clouds.

36
Suggestions of cloud infall in external galaxies
Holes punctured In the HI disk (out of the SF
region)
Velocity wiggles
NGC 6946 Boomsma 2007
37
Extraplanar gas, warps, streams, filaments

• Modeling extra-planar gas kinematics
  FraternaliBinney
• Cannot be explained by fountain
• Suggests net inflow of gas
• Similar for other galaxies

NGC 891 Osterloo et al. 2007