A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES

By Mark R. Krumholz, Nickolay Y. Gnedin. 2010

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Recent observations indicate that star formation occurs only in the molecular phase of a galaxy's interstellar medium. A realistic treatment of star formation in simulations and analytic models of galaxies therefore requires that one determine where the transition from the atomic to molecular gas occurs. In this paper, we compare two methods for making this determination in cosmological simulations where the internal structures of molecular clouds are unresolved: a complex time-dependent chemistry network coupled to a radiative transfer calculation of the dissociating ultraviolet (UV) radiation field and a simple time-independent analytic approximation. We show that these two methods produce excellent agreement at all metallicities ≳10−2 of the Milky Way value across a very wide range of UV fields. At lower metallicities the agreement is worse, likely because time-dependent effects become important; however, there are no observational calibrations of molecular gas content at such low metallicities, so it is unclear if either method is accurate. The comparison suggests that, in many but not all applications, the analytic approximation provides a viable and nearly cost-free alternative to full time-dependent chemistry and radiative transfer.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/fcfe554f8f22a4d0a7c8648a4465f7fee07deb60

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Citations in the corpus

METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, methodology): “This analytic model agrees very well with numerical simulations that follow the full time-dependent chemistry of H2 formation and dissociation (Krumholz & Gnedin 2011).”
simba: Cosmological simulations with black hole growth and feedback (2019, methodology): “As in Mufasa, we use an H2-based star formation rate, where the H2 fraction is computed based on the subgrid model of Krumholz & Gnedin (2011) based on the metallicity and local column density, with minor modifications as described in Davé et al. (2016) to account for variations in numerical…”
The impact of baryonic physics on the structure of dark matter haloes: the view from the FIRE cosmological simulations (2015, methodology): “We follow Krumholz & Gnedin (2011) to calculate the molecular fraction fH2 in dense gas as a function of local column density and metallicity.”
Absorption-line systems in simulated galaxies fed by cold streams (2011, result): “This analytic model reproduces to the first order the molecular gas fraction in nearby spirals (Krumholz, McKee & Tumlinson 2009) and metal-poor dwarfs (Fumagalli, Krumholz & Hunt 2010a) and approximates the results from numerical simula- tions (Krumholz & Gnedin 2011).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “The most obvious way to measure the IMF is to begin with field stars visible in the Solar neighborhood, an effort that began with Salpeter (1955).”
MUFASA: Galaxy Formation Simulations With Meshless Hydrodynamics (2016, methodology): “This factor appears in the rate coefficient for forming H2 on dust grains (see §2.2 of Krumholz & Gnedin 2011).”
Galaxies on FIRE (Feedback In Realistic Environments): stellar feedback explains cosmologically inefficient star formation (2013, unknown): “We follow Krumholz & Gnedin (2011) to calculate the molecular fraction fH2 in dense gas as a function of local column density and metallicity, and allow SF only from molecular gas.”
BREATHING FIRE: HOW STELLAR FEEDBACK DRIVES RADIAL MIGRATION, RAPID SIZE FLUCTUATIONS, AND POPULATION GRADIENTS IN LOW-MASS GALAXIES (2015, methodology): “Star-forming gas must be molecular, as determined by the molecular fraction calculated from the local column density and metallicity according to Krumholz & Gnedin (2011).”
Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds (2011, methodology): “Our models also include realistic cooling to temperatures 100 K, and a treatment of the molecular/atomic transition in gas and its effect on star formation (following Krumholz & Gnedin 2011).”
ON THE INTERPLAY BETWEEN STAR FORMATION AND FEEDBACK IN GALAXY FORMATION SIMULATIONS (2014, background): “Krumholz & Gnedin (2011) found that the two phase approximation predicts the H2 abundance accurately compared to full non-equilibrium radiative transfer calculations for Zg & 10−2Z .”
Not so lumpy after all: modelling the depletion of dark matter subhaloes by Milky Way-like galaxies (2017, background): “Star formation occurs in self-gravitating gas (according to the criterion in Hopkins et al. 2013) that is also molecular and self-shielding (following Krumholz & Gnedin 2011), Jeans unstable, and exceeds a minimum density threshold nsf > 1000 cm −3.”
FIRE in the field: Simulating the threshold of galaxy formation (2016, background): “Star formation occurs only in locally self-gravitating (following Hopkins et al. 2013), selfshielding and molecular (following Krumholz & Gnedin 2011), Jeans-unstable regions with densities > 1000 cm−3; gas that meets these criteria is turned into stars on its free-fall time.”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “A comparison of this method against fully time dependent chemical reaction networks by Krumholz & Gnedin (2011) finds good correspondence between the methods above metallicities of Z′ ≈ 0.01.”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, methodology): “The expression for the shielded fraction from Krumholz & Gnedin (2011) is: f (sf)shielded ≡ 1− 3 1 + 4 ψ̃a (C2) ψ̃a ≡ 0.6 τ̃a (0.01 + Za/Z ) ln(1 + 0.6 φ̃a + 0.01 φ̃2a) (C3) φ̃a ≡ 0.756(1 + 3.1Za/Z ) 0.365 (C4) τ̃a ≡ 434.8cm2 g−1 ρa ( ha + ρa |∇ρ|a ) (C5) We require f (sf)shielded > 0 for star…”
The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback (2013, background): “…inside of typical GMCs. (3) Molecular Gas: We calculate the molecular fraction fH2 c© 0000 RAS, MNRAS 000, 000–000 of all gas as a function of the local column density and metallicity following Krumholz & Gnedin (2011) and allow star formation only from the molecular gas (i.e. multiply by fH2 ).”
Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy (2014, background): “Star formation takes place in molecular, self-gravitating gas above a hydrogen number density of n H > 10 cm −3, where the molecular fraction is calculated following Krumholz & Gnedin (2011) and the self-gravitating criterion following Hopkins, Narayanan & Murray (2013).”
Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, methodology): “This model has since been used to verify equilibrium analytic models for the H2 abundance for gas with metallicity≥ 10−2 Z⊙ (Krumholz & Gnedin 2011).”
The structure of the interstellar medium of star‐forming galaxies (2011, methodology): “We then calculate the local molecular fraction in each particle as a function of column density, metallicity and also ionizing field strength from the fitting function in Krumholz & Gnedin (2011).”
Forged in FIRE: cusps, cores and baryons in low-mass dwarf galaxies (2015, background): “We follow Krumholz & Gnedin (2011) to calculate the molecular fraction fH2 in dense gas as a function of local column density and metallicity, and allow SF only from molecular gas.”