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
fra
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f ba
ryo
ns in
sta
rs
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
phas
e O
xyge
n ab
unda
nce
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