H I-TO-H₂TRANSITIONS AND H I COLUMN DENSITIES IN GALAXY STAR-FORMING REGIONS

Abstract

We present new analytic theory and radiative transfer computations for the atomic to molecular (HI-to-H 2 ) transitions, and the build-up of atomic-hydrogen (HI) gas columns, in optically thick interstellar clouds, irradiated by far-ultraviolet photodissociating radiation fields. We derive analytic expressions for the total HI column densities for (one-dimensional (1D)) planar slabs, for beamed or isotropic radiation fields, from the weak-to strong-field limits, for gradual or sharp atomic to molecular transitions, and for arbitrary metallicity. Our expressions may be used to evaluate the HI column densities as functions of the radiation field intensity and the H 2 -dust-limited dissociation flux, the hydrogen gas density, and the metallicity-dependent H 2 formation rate-coefficient and far-UV dustgrain absorption cross-section. We make the distinction between "HI-dust" and "H 2 -dust" opacity, and we present computations for the "universal H 2 -dust-limited effective dissociation bandwidth". We validate our analytic formulae with Meudon PDR code computations for the HI-to-H 2 density profiles, and total HI column densities. We show that our general 1D formulae predict HI columns and H 2 mass fractions that are essentially identical to those found in more complicated (and approximate) spherical (shell/core) models. We apply our theory to compute H 2 mass fractions and star-formation thresholds for individual clouds in self-regulated galaxy disks, for a wide range of metallicities. Our formulae for the HI columns and H 2 mass fractions may be incorporated into hydrodynamics simulations for galaxy evolution.

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

“…The free-space photodissocation rate of H 2 is 4.25 × 10 −11 G 0 s −1 , also as in Sternberg et al (2014). The accuracy of the Draine & Bertoldi (1996) formulation was recently verified in great detail by Sternberg et al (2014) using an exact calculation in the context of the Meudon PDR code. Continuous absorption by dust is especially important at densities n(H) 32 cm −3 when large hydrogen columns are required before much H 2 accumulates (Liszt 2015).…”

“…The optical depth for dust absorption is τ d = 1.9 × 10 −21 N(H) (Draine 2003) as in Sternberg et al (2014). The free-space photodissocation rate of H 2 is 4.25 × 10 −11 G 0 s −1 , also as in Sternberg et al (2014).…”

“…Following the prescription of (Spitzer 1978, see also Sternberg et al (2014)) the rate constant for H 2 -formation on grain surfaces is taken as R G = 3 × 10 −18 cm 3 s −1 √ T K where T K is the locally-computed kinetic temperature. However, the thermal balance and temperature-dependent rate constant are not of crucial importance to the H 2 -fraction.…”

Origin andz-distribution of Galactic diffuse [C II] emission

“…Therefore, we interpret the higher rate of detection at large H i intensities, not as arising from the H i gas itself, but with the probability for the presence of high density molecular gas associated with increasing H i column density. In other words, as H 2 fractions are expected to increase with increasing H i column densities (e.g., Sternberg et al 2014 In the following we use the results of the spaxel detection only to quantify the fraction of Galactic [C ii] likely to be associated with H 2 gas (see Sect. 4.2) and to differentiate it from the emission in the diffuse atomic H i gas or the WIM (see Sect.…”

H2distribution during the formation of multiphase molecular clouds

“…Since understanding the atomic-to-molecular hydrogen (HI-to-H 2 ) transition is of the highest importance for understanding the star-forming process, numerous models have been developed (e.g. Krumholz et al 2008;Sternberg et al 2014) and seem able to reproduce many of the observed constraints. While extremely useful, these models leave aside the dynamical aspects of H 2 formation.…”