The Atomic-to-Molecular Transition in Galaxies. I. An Analytic Approximation for Photodissociation Fronts in Finite Clouds

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

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In this series of papers, we study the structure of the atomic-to-molecular transition in the giant atomic-molecular complexes that are the repositories of most molecular gas in galaxies, with the ultimate goal of attaining a better understanding of what determines galaxies’ molecular content. Here we derive an approximate analytic solution for the structure of a photodissociation region (PDR) in a cloud of finite size that is bathed in an external dissociating radiation field. Our solution extends previous work, which with few exceptions has been restricted to a one-dimensional treatment of the radiation field. We show that our analytic results compare favorably to exact numerical calculations in the one-dimensional limit. However, our more general geometry provides a more realistic representation than a semi-infinite slab for atomic-molecular complexes exposed to the interstellar radiation field, particularly in environments such as low-metallicity dwarf galaxies, where the curvature and finite size of the atomic envelope cannot be neglected. For clouds that are at least 20% molecular, we obtain analytic expressions for the molecular fraction in terms of properties of the gas and radiation field that are accurate to tens of percent, while for clouds of lower molecular content we obtain upper limits. As a side benefit, our analysis helps to clarify when self-shielding is the dominant process in H2 formation, and under what circumstances shielding by dust makes a significant contribution.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/c2628d13f694c0fcd82ad520dbb8af431b540933

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Citations in the corpus

A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES (2010, methodology): “The Analytic Model The analytic model to which we compare the simulations is based on the work of Krumholz et al. (2008), Krumholz et al. (2009b), and McKee & Krumholz (2010), hereafter known as the KMT model.”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, background): “On small scales of individual molecular complexes it is primarily the cosmic dust abundance and the interstellar FUV radiation that control the atomic-to-molecular transition (e.g., Elmegreen 1993; Krumholz et al. 2008, see Stahler & Palla 2005 for pedagogical review).”
METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, background): “Krumholz et al. (2008, 2009b) and McKee & Krumholz (2010) show that these processes produce a molecular fraction that depends primarily on the surface density and metallicity of the galactic disk, and relatively little on any other parameters.”
THE SPATIALLY RESOLVED STAR FORMATION LAW FROM INTEGRAL FIELD SPECTROSCOPY: VIRUS-P OBSERVATIONS OF NGC 5194 (2009, background): “First, the fraction of the gas in molecular form is set by the balance between the formation of H2 in the surface of dust grains, and the dissociation of molecules by the far-UV continuum in the Lyman-Werner bands (Krumholz et al. 2008, 2009a).”
THE ARECIBO LEGACY FAST ALFA SURVEY: THE GALAXY POPULATION DETECTED BY ALFALFA (2012, background): “The relative proportions of molecular and dense atomic gas in giant molecular clouds depend on the cloud column density and metallicity (Krumholz et al. 2008), and the clouds could even be primarily atomic if the metallicity is sufficiently low (Ostriker et al. 2010).”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “H I balance in these clouds utilizing the analytic model of Krumholz et al. (2008); ,2009a) and McKee & Krumholz (2010) which treats the balance between photodissociation of H2 molecules by Lyman–Werner band photons against the formation of molecules on grains.”
THE DARK MOLECULAR GAS (2010, result): “We find that the column density to the transition region is a function ofG′0/nc consistent with previous investigations (e.g., Sternberg 1988; Hollenbach & Tielens 1999; Wolfire et al. 2008; Krumholz et al. 2008; McKee & Krumholz 2010).”
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.”
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.”
THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H i AND H2 COLUMN DENSITIES (2008, background): “A second complication for this data set is that the assumption we make in our model that clouds are subjected to a relatively uniform background dissociating radiation field (cf. Krumholz et al. 2008) may not be appropriate in this context.”
Star formation in semi-analytic galaxy formation models with multiphase gas (2015, methodology): “…et al. (2009b) presented analytic models for the formation of H2 as a function of total gas density and metallicity, supported by numerical simulations with simplified geometries (Krumholz et al. 2008, 2009a), emphasizing the importance of metallicity as a controlling parameter in H2 formation.”
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).”
Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, methodology): “In our simulations withH2 we incorporated a link between star formation and MCs by modifying the probability of a star forming to include the local fraction ofH2.”
The EAGLE project: Simulating the evolution and assembly of galaxies and their environments (2014, background): “…metallicity is increased due to the higher formation rate of molecular hydrogen, the increased cooling due to metals and the in- c© 0000 RAS, MNRAS 000, 000–000 creased shielding by dust (e.g. Schaye 2001, 2004; Pelupessy et al. 2006; Krumholz et al. 2008; Gnedin et al. 2009; Richings et al. 2014).”