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Cosmic Star Formation History:the role of diffuse accretion

Romain TeyssierCEA Saclay

http://www.projet-horizon.fr

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Physics:• Cooling by H, He + metals and heating by Haardt & Madau UV background• Multiphase ISM as a polytropic equation of state• Star formation as a sub-grid model• Supernova driven winds• AGN driven winds ?• Star bursts ?

Many problems:• Overcooling• Angular momentum• Missing satellites

Numerical issues:• SPH versus AMR• Resolution in mass• Resolution in space and time

Next generation of galaxy formation models: MHD ?

Cosmological simulations of galaxy formation

M82 M81

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Select star forming regions with: Form stars at the local rate:

with

• t0= 1-10 Gyr (Kennicutt 1998)

• α = 0.2-0.02 (Krumholz & Tan 2007)

• n0= 0.01-0.1 H/cm3 (Martin & Kennicutt 2001)

Numerical implementation :spawn constant mass « star particles» drawnfrom a Poisson random process

Star formation modeling

Caveat: this phenomenological approach is valid only for kpc scales.Discs are very smooth (polytrope) and poorly resolvedNext generation simulations: molecular cooling up to 104 H/cc, sub-cell

model for starbursts, MHD…

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« Quiescent » supernovae driven winds galactic wind versus galactic fountain

1010 M 1011 M

1010 M halo(various λ and t*)

Dubois & Teyssier, 2007, in press.Wind formation only for Vcirc < 80km/s:

Wind break-outafter 1 to 3 Gyr.

Wind efficiencybelow 10%

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10000h-1kpc 2500h-1kpc 630h-1kpc 160h-1kpc

Dark matterdensity

Gastemperature

Gasdensity

Stellardensity

LCDM universeLbox=10 Mpc/hFinal redshift z=3

comoving

RAMSES code5123 particles1/2 billions cells256 processors350 hours

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Ionizing radiation flux from young stars and quasars heats up baryons before they collapse:Pressure support preventing gas from collapsing into low-mass DM halos: M200<MF (Gnedin 2000)Inefficent atomic cooling: T200< Tmin ≈104 K Star forming halos: M200 > Mmin=max(MF,Mcool)

A minimal mass for star forming halos ?

Diffuse gas accretion (M200<Mmin) onto star forming halos (M200>Mmin) :

Press & Schechter (1974) mass function

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Accretion

Cooling

Star formation

Winds COLD DISCSIGM

HOTGAS

STARS

4 mass transfer branches :- Diffuse accretion into halos- Cooling into discs- Star formation within discs- Winds from star forming discs

Baryons logistics using the halo model4 baryon phases :

- Diffuse IGM (Lyα forest)- Hot gas in hydrostatic halos- Cold gas in rotating discs- Stars

Halo model for baryons: a simple analyticalmodel based on the Extended Press andSchechter halo statistics and various SAM-likeapproximations (Rasera & Teyssier 2006).

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(for all halos with Vcirc<200 km/s)

Delay the peak Lower the amplitude

Hierarchical model predictions

Wind efficencyStar formation time scale

(t* ≈ t0/3)

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RAMSES simulations with box size: 10 h-1MpcAnalytical model with finite resolution effectsRasera & Teyssier (2006)

Analytical model and simulations

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Observations : IR, UV, Radio (compilation by Elbaz 2005)Isocontours for z=0 and z=3 are orthogonal.« Best fit » for t0=8 Gyr and ηw= 1 (Rasera & Teyssier 2006)We need very strong supernovae driven winds !

Star formation history in the universe

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Heavens et al 2004: stellar population of 100000 galaxies (SDSS) Earlier SFH for larger galaxies : anti-hierarchical ? Fast decline of SFH for large objects at low z⇒ SF shutdown, bimodal accretion, PdV heating, AGN feedback ?

Star formation shut-down above a critical mass ?

Star formation history in individual halos

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MareNostrum galaxy formation project

GADGET team: G. Yepes, S. Goettlober, M. Hoeft, A. Khalatyan…RAMSES team: R. Teyssier, D. Aubert, E. Audit, J. Devriendt, C. Pichon…with strong support from BSC and IDRIS.

GADGET RAMSES SPH z=5.7 AMR

50 h-1 Mpc box with 10243 particles and 4 billion AMR cells10243 base grid +5 levels of refinement (smallest cell is 2 kpc physical)N body + gas, cooling, star formation, polytrope, supernovae blast waves, metals

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Bimodality: cold streams or hot shocks?

Birnboim & Dekel (2003)Kravtsov (2003)Keres et al. (2005)Dekel & Birnboim (2006)

SSDS data (from Baldry et al. 2005)

MareNostrum run

Hypothesis: the galaxy star formation historydepends on the geometry of the diffuse accretion.Filaments feed directly fresh gas into the disc.Hot shocks stop cooling and therefore gas accretion.

Shock stability: tcool(ρvir) ~Rvir/Vvir

Filament survival: tcool(ρf) ~Rvir/Vvir

Density enhancement: ρf Tf ~ρvirTvir

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Accretion-weighted histograms

60% of accretionin cold phase

40% of accretionin hot phase

20% of accretionin cold phase

80% of accretionin hot phase

Ocvirk, Pichon & Teyssier (2008) in preparation

A proxy for detecting hot shocks: accretion-weighted histogram @ 0.2xRvirA proxy for detecting cold streams: accretion-weighted histogram in [0.2-1]xRvirCritical temperature T0=2.5x105 K (as in Keres et al. 2005)

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Bimodality in smooth accretion flows

Metallicity of the hot phase is thekey parameter !

Transition masses for cold streams

Transition masses for hot shocks

Hypothesis: star formation in a galaxy is related to theproperties of the diffuse (filamentary) gas accretion