THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H i AND H2 COLUMN DENSITIES

By Mark R. Krumholz, C. McKee, Jason Tumlinson. 2008

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Gas in galactic disks is collected by gravitational instabilities into giant atomic–molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic-to-molecular hydrogen in complexes is, therefore, a significant problem in star formation and galactic evolution. In this paper, we use the model of H2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic-to-molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to a good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ∼10 M☉ pc−2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/74bf4dbb64a1b05f8a29c9433fbeaabc3f7669dc

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Citations in the corpus

The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “…for the attenuation of the radiation field as a function of column density (Draine & Bertoldi 1996), and for the amount of shielding required for a transition from H i to H2 in either slab (Sternberg 1988) or spherical (Elmegreen 1993; Krumholz et al. 2008, 2009a; McKee & Krumholz 2010) geometry.”
EVOLVING GRAVITATIONALLY UNSTABLE DISKS OVER COSMIC TIME: IMPLICATIONS FOR THICK DISK FORMATION (2011, methodology): “Following progress made by Krumholz et al. (2008, 2009a), McKee & Krumholz (2010) have analytically approximated the molecular fraction of the gas, fH2 as a function of metallicity and surface density.”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “Krumholz et al. (2009) focused on shielding of H2 inside individual atomic– molecular complexes, whereas Ostriker et al. (2010) examined the interplay between large–scale thermal and dynamical equilibria.”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, methodology): “Krumholz et al. (2009) revisited the SFR–gas relation, considering the dependence on atomic and molecular components of Σgas, metallicity, and clumping of the gas.”
How Stellar Feedback Simultaneously Regulates Star Formation and Drives Outflows (2015, background): “For the gas to be selfshielding, Σg & 10 M⊙ pc −2 ( Z Z⊙ )−1 ≡ Σself−shield (34) (Krumholz et al. 2009a).”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, background): “…KS relation on gas metallicity, interstellar FUV radiation, and other properties of galaxies are in their early stages (e.g., Bigiel et al. 2008; Krumholz et al. 2009a; Rafelski 2009), it is clear that such strong dependences can have important implications for our understanding of galaxy…”
Ingredients for 21 cm Intensity Mapping (2018, methodology): “We then compute the H2 fraction, fH2 = MH2/MNH employing the KMT model (Krumholz et al. 2008, 2009; McKee & Krumholz 2010; see also Sternberg et al. 2014), which we briefly review here.”
A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES (2010, methodology): “Driven by these observations, analytic models (Blitz & Rosolowsky 2006; Krumholz et al. 2009c), semi-analytic models (Obreschkow & Rawlings 2009; Obreschkow et al. 2009; Fu et al. 2010; Dutton et al. 2010), and simulations (Pelupessy et al. 2006; Robertson & Kravtsov 2008; Pelupessy & Papadopoulos…”
ON THE INTERPLAY BETWEEN STAR FORMATION AND FEEDBACK IN GALAXY FORMATION SIMULATIONS (2014, background): “Krumholz et al. (2008), Krumholz et al. (2009a), and McKee & Krumholz (2010) developed a model for the abundance of H2 based on radiative transfer calculations of idealized spherical giant atomic–molecular complexes subject to a uniform and isotropic Lyman-Werner (LW) radiation field.”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, background): “Similarly, observational (Bigiel et al. 2008) and theoretical work (Krumholz et al. 2009b; Ostriker & Shetty 2011) suggest that the SFR index may be superlinear in high-surface density environments.”
ENZO: AN ADAPTIVE MESH REFINEMENT CODE FOR ASTROPHYSICS (2013, background): “Cells are examined every root grid timestep, ∆t0, and cells at the highest level that exceed the H2 limit form star particles with masses proportional to the inferred mass of molecular hydrogen in the star-forming region (Krumholz et al. 2008, 2009; McKee & Krumholz 2010).”
SIMULATION OF THE COSMIC EVOLUTION OF ATOMIC AND MOLECULAR HYDROGEN IN GALAXIES (2009, background): “…and neutral hydrogen mass MH = MHI+MH2 of a galaxy into its atomic (HI) and molecular (H2) component based on the observed and theoretically confirmed relation between local gas pressure and local molecular fraction (Elmegreen 1993; Blitz & Rosolowsky 2006; Leroy et al. 2008; Krumholz et al. 2009).”