The Other Side of Galaxy Formation: the gas

content of galaxies over cosmic history

The Other Side of Galaxy Formation: the gas

content of galaxies over cosmic history

rachel somervilleSTScI/JHURutgers

rachel somervilleSTScI/JHURutgers

motivationmotivation Improved constraints on HI and H2 content of

nearby galaxies (THINGS, GASS, COLDGASS, ALFALFA, HIPASS)

Theoretical insights into how cold gas is converted into stars

Upcoming facilities that will measure HI and H2 content of large samples of galaxies at high redshift (ALMA, LMT, MeerKAT, ASKAP, SKA)

Large samples with HST/WFC3 imaging to z~8 (e.g. CANDELS)

looking forward to JWST…

Improved constraints on HI and H2 content of nearby galaxies (THINGS, GASS, COLDGASS, ALFALFA, HIPASS)

Theoretical insights into how cold gas is converted into stars

Upcoming facilities that will measure HI and H2 content of large samples of galaxies at high redshift (ALMA, LMT, MeerKAT, ASKAP, SKA)

Large samples with HST/WFC3 imaging to z~8 (e.g. CANDELS)

looking forward to JWST…background: CANDELS image (Koekemoer et al. 2011)

z=5.7 (t=1.0 Gyr)z=5.7 (t=1.0 Gyr)

z=1.4 (t=4.7 Gyr)z=1.4 (t=4.7 Gyr)

z=0 (t=13.6 Gyr)z=0 (t=13.6 Gyr)

use large volume, relatively low resolution collisionless N-body simulations to trace growth of large scale structure (DM merger tree)

attempt to understand small-scale physical processes (e.g. star formation, BH growth) using specialized, high resolution hydrodynamic simulations

distill the relevant physics and include via ‘sub-grid’ recipes

use large volume, relatively low resolution collisionless N-body simulations to trace growth of large scale structure (DM merger tree)

attempt to understand small-scale physical processes (e.g. star formation, BH growth) using specialized, high resolution hydrodynamic simulations

distill the relevant physics and include via ‘sub-grid’ recipes

CosmologicalSimulations: Hybrid semi-analyticApproach

galaxy formation in a CDM Universe

galaxy formation in a CDM Universe

gas is collisionally heated when perturbations ‘turn around’ and collapse to form gravitationally bound structures

gas in halos cools via atomic line transitions (depends on density, temperature, and metallicity)

cooled gas collapses to form a rotationally supported disk

cold gas forms stars, with efficiency a function of gas density (e.g. Schmidt-Kennicutt Law)

massive stars and SNae reheat (and expel?) cold gas

gas is collisionally heated when perturbations ‘turn around’ and collapse to form gravitationally bound structures

gas in halos cools via atomic line transitions (depends on density, temperature, and metallicity)

cooled gas collapses to form a rotationally supported disk

cold gas forms stars, with efficiency a function of gas density (e.g. Schmidt-Kennicutt Law)

massive stars and SNae reheat (and expel?) cold gasWhite & Rees 1978; Blumenthal et al. 1984; White & Frenk 1991

the old standard: use Kennicutt relation with surface density cutoff:

the old standard: use Kennicutt relation with surface density cutoff:

•rss+08: fixed surface density crit ~ 3-6 Msun/pc2

• Croton et al./de Lucia et al.: radially dependent crit based on Toomre Q; crit ~ Vc/r for flat rotation curve

.

Bigiel et al. 2008

THINGS:star formationcares about thedensity of molecularhydrogen;almost no correlation withHI

Krumholz, McKee & Tumlinson 2008a,b, 2009McKee & Krumholz 2010; Krumholz & Dekel 2011

Molecular hydrogen formation

Molecular hydrogen formation

H2 fraction depends on gas density, amount of dust (metallicity) and intensity of UV radiation

H2 fraction depends on gas density, amount of dust (metallicity) and intensity of UV radiation

Z=1.00.30.10.010.001

see also Gnedin & Kravtsov 2010;Robertson & Kravtsov 2009 Kuhlen et al. 2011

Krumholz, McKee & Tumlinson 2008a,b, 2009

Molecular hydrogen based SF recipe

Molecular hydrogen based SF recipe

stars form from H2 with nearly constant efficiency below g~100 Msun/pc2

Possibly steeper slope for “starbursts”?

stars form from H2 with nearly constant efficiency below g~100 Msun/pc2

Possibly steeper slope for “starbursts”?

implementation in SAMimplementation in SAM assume that total gas radial density

distribution within each disk is described by an exponential; rgas propto rstar

dust-to-gas propto cold-phase metallicity; IGM “pre-enriched” to 10-3 Zsun by Pop III stars

compute f(H2) and corresponding SFRD at each timestep (do not track formation/consumption/destruction)

assume that total gas radial density distribution within each disk is described by an exponential; rgas propto rstar

dust-to-gas propto cold-phase metallicity; IGM “pre-enriched” to 10-3 Zsun by Pop III stars

compute f(H2) and corresponding SFRD at each timestep (do not track formation/consumption/destruction)

log (Mh/Msun)=10.0

Kennicuttnew recipe

log (Mh/Msun) =11

log (Mh/Msun) =12

atomicfraction

molecularfraction

observationsFrom THINGS, COLDGASS

z=0

molecularfraction

observationsfrom THINGS(z=0)

total cold gas, newHI new model

total coldgas, Kenn

H2, new model

‘quiescent’ galaxy at z=2: what alma will see

‘quiescent’ galaxy at z=2: what alma will see

G. Popping, S. Trager, J.P. Perez-Beaupuits, M. Spaans, rss:

combine SAM predictions with PDR chemical model (Meijerink & Spaans 2005)3D radiative transfer and line tracing (see P.-B. et al. 2011)

CO J=3-2 CO J=6-5

[C I] 492 GHz [C II] 158 m

Caviglia & rss in prep; see also Fontanot et al. 2009, Dave’ et al. 2011

problem: low mass galaxies form too early (stellar fractionsare too high, LF & MF too steep at high-z)

low mass galaxies are ‘too quiescent’, at least at z<2

C&S

what if we make the fraction of gas that is eligible for SF anarbitrary function of halo mass and redshift?

C&S in prep

“thinning” model

“thinning” model – simultaneously solves all ‘downsizing’ problems!

metallicity-dependent H2-based SF model

new SFrecipedoes NOTsolve the ‘downsizing’problem!

summarysummary semi-analytic cosmological models can now

track molecular and atomic gas separately enables predictions of gas properties at high-z,

to be tested with upcoming facilities, and implementation of more physical H2-based SF recipes – important implications for low mass and z>6 galaxies

models predict gas fractions and molecular fractions higher in the past

explaining properties of low-mass galaxies so far remains a challenge for LCDM-based models

semi-analytic cosmological models can now track molecular and atomic gas separately

enables predictions of gas properties at high-z, to be tested with upcoming facilities, and implementation of more physical H2-based SF recipes – important implications for low mass and z>6 galaxies

models predict gas fractions and molecular fractions higher in the past

explaining properties of low-mass galaxies so far remains a challenge for LCDM-based models

energy from SNae and massive stars assumed to drive large-scale outflows which remove cold gas from the disk, thus making it (temporarily) unavailable for SF

energy from SNae and massive stars assumed to drive large-scale outflows which remove cold gas from the disk, thus making it (temporarily) unavailable for SF

Large-scale galactic outflows

Large-scale galactic outflows

=1 “momentum driven”=2 “energy driven”

Fiducial

Gnedin & Kravtsov 2010; see also Robertson & Kravtsov 2008

dust-to-gas

UV field

Kennicutt relation at z=6

low-mass halos are really bad at making stars

low-mass halos are really bad at making stars

DM halos

stars

• abundance matching indicates stellar fractions decline sharply with decreasing halo mass below Mh~1012 Msun

• very low baryon fractions corroborated by kinematics of dwarf galaxies

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Moster, rss et al. 2009, Behroozi et al. 2010

archeological downsizingarcheological downsizing

data: Gallazzi et al. 2007

Fontanot et al. 2009

stellar populations in low mass model galaxies are too old, downsizing is too weak

partly, but not wholly, due to biases intrinsic to age estimates from Balmer lines (see Trager & rss 2008)

stellar populations in low mass model galaxies are too old, downsizing is too weak

partly, but not wholly, due to biases intrinsic to age estimates from Balmer lines (see Trager & rss 2008)

gas

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n ab

unda

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Arrigoni, Trager & rss 2011

low mass galaxiesbecome enrichedtoo early

blue lines: z=0 relationfrom Tremonti et al. 2004blue symbols: observationsat various redshiftsred dots: central model galaxiesgray: all model galaxiesgreen: median for model galaxiesmagenta: model galaxies z=0

summary of problems with low-mass galaxies

summary of problems with low-mass galaxies

too numerous at high-z; low-mass halos at high-z have stellar fractions that are too high

specific star formation rates too low at all redshifts where we can measure them

stellar population ages at z=0 too old

become chemically enriched too early

too numerous at high-z; low-mass halos at high-z have stellar fractions that are too high

specific star formation rates too low at all redshifts where we can measure them

stellar population ages at z=0 too old

become chemically enriched too early