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Star Formation Star Formation in Cosmological Simulations:in Cosmological Simulations:the Molecular Gas Connectionthe Molecular Gas Connection
Kostas Tassis
Jet Propulsion LaboratoryCalifornia Institute of Technology
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Co-starringCo-starring
Andrey Kravtsov
Nick Gnedin
Gnedin, Tassis & Kravtsov 2009 ApJ, 697, 55 /arXiv:0810.4148
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Star Formation in cosmological simulationsCosmological Simulations: • ISM physics not followed in detail• Sub-grid SF modeling typically based on
observed scaling relations, tied to cold gas (HI+H2)
Observations of high z galaxies:– Probe molecular content, star formation
(ALMA,JWST)– Simulations must match this advance
and increase fidelity of simulated galaxies
Kennicutt 1998
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Use ART (Adaptive Refinement Tree) N-body+gas+SF+RT:• 6 Mpc comoving box
• 50 pc resolution in high-redshift galaxies
• Mass resolution mdm=106 Msun, mgas=105 Msun
StartingStarting PointPoint
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StartingStarting PointPoint
Use ART (Adaptive Refinement Tree) N-body+gas+SF+RT:• 6 Mpc comoving box
• 50 pc resolution in high-redshift galaxies
• Mass resolution mdm=106 Msun, mgas=105 Msun
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StartingStarting PointPoint
Use ART (Adaptive Refinement Tree) N-body+gas+SF+RT:• 6 Mpc comoving box
• 50 pc resolution in high-redshift galaxies
• Mass resolution mdm=106 Msun, mgas=105 Msun
• Optically Thin Variable Eddington Tensor Approximation (OTVET) for following time-dependent and spatially-variable RT
• Cooling rates and ionization/chemical balance are computed “on the fly”
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How Molecular Clouds FormHow Molecular Clouds Form
H2 forms on the surface of dust grains.
Self-shielding starts the process, dust shielding finishes it.
Young stars
Molecular gas
Atomic gas Dust
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HH22 FormationFormation Model: gas + dustModel: gas + dust
• “Primordial” hydrogen balance (gas-phase reactions)
• Adding dust heuristically
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HH22 FormationFormation Model: parametersModel: parameters
Formation rate:
• Dust/Gas ~ Z• Molecular gas is inhomogeneous, clumping factor
~ 10 for typical clumpy molecular cloud models (e.g.: Padoan et al. 1997; Ostriker et al. 2001)
Shielding factors:
a la Draine & Bertoldi (1996) see also Glover& Mac Low(2007)
• Parameters & to be calibrated
Wolfire et al. (2008)
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• H2 fractions in translucent clouds have been measured by Copernicus & FUSE space missions (Tumlinson et al 2002, Rachford et al 2002, Gillmon et al 2006, Wolfire et al 2008)
TrainingTraining thethe ModelModel
• HINSA measurements of HI fractions (Goldsmith & Li 2005)
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StarStar FormationFormation
(Krumholz & Tan 2006)
Specific SFR per local free-fall time is not sensitive to the environment (both normal galaxies and starbursts), and is about 1-2% in molecular gas (star formation is slow).
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ThermodynamicsThermodynamics
Automatically get 3+phases:
• Hot coronal gas
• Warm neutral and ionized medium
• Cold neutral and molecular gas
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Atomic-to-molecularAtomic-to-molecular TransitionTransition
Transition between atomic and molecular phases
• very sharp
• scales with metallicity
Z=1.0 Z=0.3 Z=0.1
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Simulation
KennicuttKennicutt PlotsPlots
Reality (THINGS)Bigiel et al. 2008
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KennicuttKennicutt PlotsPlots
Simulation Reality (THINGS)Bigiel et al. 2008
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KennicuttKennicutt PlotsPlots
Simulation Reality (CO)Bigiel et al. 2008
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KennicuttKennicutt PlotsPlots
Simulation Reality (THINGS+CO)Bigiel et al. 2008
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SummarySummary
• SF in cosmological simulations needs to be tied to molecular hydrogen – just like in nature!
•Transition from atomic to molecular gas is very sharp – lack of dependence of SFR on HI surface density in Kennicutt-like correlations.
•In the zeroth order, the SF density threshold scales inversely with metallicity, nSF ≈ 30/Z cm-3.
•Since dust-to-gas ratio depends on the metallicity, it constitutes a feedback Since dust-to-gas ratio depends on the metallicity, it constitutes a feedback effect that needs to be accounted for in cosmological simulations and galaxy effect that needs to be accounted for in cosmological simulations and galaxy evolution models.evolution models.