Transcript Growth of Galaxies in Cosmological Simulations (30 minutes)

The EVLA Vision: Galaxies Through Cosmic Time
Dec. 2008
How do Galaxies Get Their Gas?
Dušan Kereš
Institute for Theory and Computation Harvard-Smithsonian CFA
Collaborators: Neal Katz, David Weinberg, Romeel
Davé, Mark Fardal, Lars Hernquist, T. J. Cox, Phil Hopkins
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?
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 -> can
provide high SFRs.
• Depletion of this gas can then slow down the
star formation
• Could explain some of the observed trends
• Let’s check if there is enough “galactic” gas at
high redshift
* *
The state of dense gas
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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 z=0.
A vast majority (~80%) of stars
form after z=2 (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.
Prochaska&Wolfe ‘08
z=0
Is there an evidence for gas
supply in our neighborhood?
Milky Way
• MW’s SFR 1-3M_sun/yr (Kennicutt ‘01)
• MW’s 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
HVCs around MW
• Most direct measure of galactic infall
• Many clouds with inwards velocities, net infall from
known clouds > 0.25M/yr
• Hard to explain by the galactic fountain
• Low metallicities
• Similar net infall rates from clouds around other galaxies
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Van Woerden et al. 2004
• Conclusion: Accretion of gas from the
intergalactic medium is ongoing at all
epochs.
The Theory
Standard Model
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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.
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Tvir=106(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 –> the (spiral) galaxy.
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Mergers of disks can later produce spheroids.
The base for simplified prescriptions used in SemiAnalytic models – SAMs (e. g. White & Frenk 1991).
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Tvir
NASA/WMAP
Structure formation
Gas heating
Gas cooling
Dynamics-merging
Star-formation
FEEDBACK
Earliest
Observation
How do galaxies form and evolve?
Complex and nonlinear: Simulations
needed
Today
NOAO
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 -> we have ability to follow fluid (particles) in
time and space.
Global Accretion
Kereš et al. 2008
• 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 z=1
• 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)
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
Shocked Tmax = ?
IGM
Galactic gas
Evolution of gas properties of
- Empirical division of
accreted particles
2.e5K, but results are
robust for 1.5-3e5K
- The gas that was not
heated to high
temperatures -> COLD
MODE ACCRETION
- > Accretion from cold
filaments
- The gas that follows the
standard model -> HOT
MODE ACCRETION
- > Accretion from cooling
of the hot halo gas
Kereš et al. 2005
Katz, Keres et al. 2002
250000K
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
The nature of cold and hot
modes
Low mass halos are not
virialized
• Halo gas (excluding
galactic gas), within Rvir
• Low mass halos are not
virialized
• Transition: Mh=1011.4 M⊙
• Approximate description
by Birnboim&Dekel 2003
• Post shock cooling times
are shorter than
compression time at Rvir
when Mh~1x1011 M⊙
• More gradual transition in
our case because cold
filaments enhance the
density profile
Kereš et al 2008
Low mass halos
Low mass halos
• High-z well
defined
filamentary
accretion
• Low-z ->
Tvir is lower,
filaments are
warmer and
larger than
the halo size
High-z accretion
QuickTi me™ and a
TIFF ( LZW) decompressor
are needed to see thi s pi ctur e.
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QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Kereš 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
< 1kpc resolution, m_p ~1.e6 M_sun
Kereš et al 2008
Zoom to 0.8Rvir
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z~>2 cold mode can supply the central galaxy and satellite galaxies
efficiency to supply galaxies directly declines with time
z~1 cold mode less efficient in massive halos; lower density contrast ->
easier to disrupt
• It can supply satellites directly but supply to the center is limited
and often goes trough cold clumps
Low-z accretion
• Drastic accretion change over time,
factor of ~30 from z=4 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 z~0, a fraction of massive halos,
around MW mass is able to cool
the hot gas
Kereš et al. 2008
Red - hot mode
Blue - cold mode
Recent & future simulations
New simulations
5kpc/h physical
Z~2.6
110/h pc physical res.
M_p~1.e5M_sun
M_h~1.3e11M_sun
Yellow Tvir
Blue ~ 1e4K
300/h kpc ~ 2Rvir
1.2/h Mpc cmv.
z~2.6 25/h kpc (physical)
Halo mass at z=0, 7x1011M_sun
Gas particle mass ~105M_sun
QuickTime™ and a
mpeg4 decompressor
are needed to see this picture.
Zoom onto a ~Rvir region, with
the lowest overdensity moving from 200 to 1500
QuickTime™ and a
decompressor
are needed to see this picture.
Can we detect the smoothly
accreting gas with EVLA?
(very preliminary …)
z=2.5
z=2.0
z=1.5
400kpc region, 1pixel=1kpc, approx 5” at 40Mpc
Log
n_HI
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18
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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 z=1
• Lower mass galaxies are embedded in thick
filaments
• Need to work on kinematic signatures
z=1.0
z=0.5
z=0
400kpc region, 1pixel=1kpc, approx 5” at 40Mpc
EVLA range z=0-0.5
• 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 >1.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.
Where should an idealized HI
telescope look?
• Higher redshift -> more coherent structure
• Higher mass galaxies -> 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
• z=1 is already an interesting regime.
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 > 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.