Download - 1 Lessons from cosmic history Star formation laws and their role in galaxy evolution R. Feldmann UC Berkeley see Feldmann 2013, arXiv:1212.2223 1

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Lessons from cosmic Lessons from cosmic historyhistory

Star formation laws and their role in galaxy Star formation laws and their role in galaxy evolutionevolution

R. FeldmannUC Berkeleysee Feldmann 2013,

arXiv:1212.2223 1

2Hubble Fellowship Symposium, March 2013


M31


30 Doradus•SF and Galaxy evolution strongly

linked•How to move forward without

solving SF?


Star formation “law” = empirical relation between SF and ISM

Abstracts from details of small scale SF physics & feedback

Essential ingredient in theoretical models of galaxy evolution!

Main applications:•use as “effective model of SF” on super-

GMC scales

•constrain small scale physics


0

-1

-2

-3

-4

•strong correlation over orders of magnitude of ISM surface densities

• reasonably tight

•slope ~1, tdep ~ 2.3 Gyr

•deceptively simple: the more H2 the more SF

H2 - SF relation:

A simple “effective” model of SF in the local Universe !

Bigiel+11In the local Universe


At high redshift (out to z~2)

•galaxies in the “main sequence” of SF follow a ~linear relation with a ~Gyr depletion time

• interacting/merging galaxies are offset

•potentially observational systematics

Genzel+2010

•SF tracers (IR cirrus)

•CO/H2 conversion factor

•quadratic relation?Determine H2 - SF relationship

indirectly?

H2 - SF relation:


Part I. Testing different star formation laws

•linear vs quadratic law•cosmic SFH & evolution of global galaxy properties

Outline

Part 2. Re-Evaluating Galaxy Evolution

•the role of gas accretion, metal enrichment and outflows

•galaxy evolution as an equilibrium process


Krumholz & Dekel 2012

Accretion

gas. outflows

Four components per halo

DM halo (Mhalo)exponential gas disk (Mg)

stars (Mstar)

metals (MZ)

Molecular fraction

a la Krumholz+09

Star formationA chosen SF

law

Feldmann MNRAS subm.,see also Bouche+1,

Accretion rates

Mass evolution

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Hubble Fellowship Symposium, March 2013

Star formation

11 observation-based, e.g. Bigiel+08,11

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theory-based, e.g., Ostriker & Shetty 2011, Faucher-Giguere+2013

“linear”

“quadratic”

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Cosmic SFH for a linear H2 - SF relation

Behroozi+12

Bouwens+12

model predictions (limit MUV < -17.7)

model predictions (arbitrarily faint)

H2 based SF

cold gas based SF

H2 based SF

cold gas based SF

Feldmann (MNRAS subm.)

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Cosmic SFH for a quadratic H2 - SF relation

Hubble Fellowship Symposium, March 2013 13

from Bouwens et al. 2012

not dust corrected

The cosmic star formation rate density (SFRD)

•High z observations: SFR ≪ gas accretion rate onto halos

•Models: often SFR ~ gas accretion rate even at fairly high z

dust corrected

gas accretion

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Superlinear SF law in many models (exponent ~1.4 - 2)

•more SF in high density gas => short depletion time

•overall SFR of a galaxy dominated by high density regions

•SF can catch up with the gas accretion rate

Why do models often overpredict SFR?

Linear SF law with ≳ Gyr depletion time

•depletion time long compared with accretion time at high z

•SF cannot catch up with gas accretion rate at high z

•accretion time ~ dynamical time ~ fraction of Hubble time

z~10: tacc~2x108 yr z~5: tacc~5x108 yr

•gas depletion time - depends on SF law!

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• linear law in good agreement with observations at all z

•quadratic law underpredicts gas-to-stellar fractions at high z

Gas fractions

z=0: Saintonge+11z~0.5-2.5: Magdis+12

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• linear law in good agreement with observations at all z, except in low mass galaxies at low z

•“frozen” mass-metallicity relation above z~2 in the quadratic case

Metallicity

z~0: Tremonti+04, z~2-4: Maiolino+08

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•Linear H2 - SF relation in good agreement with observations

•cosmic SFH

•mass-metallicity relation

•gas-to-stellar mass ratios

•high z UV luminosity function, ...

•Quadratic H2 - SF relation in disagreement

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• inflow of low Z gas from IGM

•outflows of enriched gas from ISM

•enrichment of the ISM following SF

Under which circumstances does Z remain constant?

!

stellar yieldrecycling fraction

gas ejection fraction

IGM metallicityratio SFR / gas accretion rate

Metallicity:

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Let , be small, and

Linear Stability Analysis

• , i.e.,

•galaxies should approach equilibrium metallicity

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Baryonic state of a galaxy ( Z, fg, fs )

then there is a (linearly) stable equilibrium that corresponds to a particular metallicity, gas fraction and stellar fraction of the galaxy.

Mg/Mhalo Ms/MhaloMZ/Mg

Given


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fg

Z, fs

r~0r~0.1r~1

•Ratio r determines the fundamental galaxy properties at any given time

•predicts strong correlations between Z, fg, and fs, and between Z, SFR, Ms, i.e., fundamental mass-metallicity relation

•Evolution of a galaxy along 1-d ``world line’’ in the baryonic state space

high z: tacc ≪ tdep

low z: tacc ~ tdep

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Differs from “equilibrium” a la Dave et al.

•galaxies are in “equilibrium” at low z

•out of “equilibrium” at high zIn this picture:

However:•no need for galaxies to have to be in equilibrium

•expect at high z:

• implies fg ~ 0

e.g., Finlator & Dave 2008, Dutton et al. 2010, Bouche et al. 2010, Dave et al. 2012

inflow rate of gas = outflow rate of gas + star formation rate

“Equilibrium condition”better: steady state

!

~1

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Zeq, fg,eq, fs,eq depend on r

Galaxy evolution is a sequence of (quasi-)equilibria in the baryonic state space driven by the (gradually) changing cosmic accretion rate.

The fundamental role of the star formation law

• functional form of SF law => equilibrium properties of galaxies

•evolution caused by the modulation of the accretion rate

Zeq, fg,eq, fs,eq ( accretion rate, adopted SF - gas relation ) baryonic physicsgravity

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Conclusions

1.Galaxy evolution studies rely on SF relations as an “effective theory of SF”

2.Functional form debated (observational systematics)

3.Predictions based on a linear relation in agreement with observations

4.Evolution of many global galaxy properties determined by • functional form of the SF relation (baryonic physics)

• matter accretion rate (gravity)

1.Galaxy evolution ~ a succession of (quasi-)equilibria driven by changes in the cosmic accretion rate

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Thank you

Download - 1 Lessons from cosmic history Star formation laws and their role in galaxy evolution R. Feldmann UC Berkeley see Feldmann 2013, arXiv:1212.2223 1

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