Metallicity Dependence of the H/H2and C+/C/CO Distributions in a Resolved Self-regulating Interstellar Medium
Journal
The Astrophysical Journal
Journal Volume
920
Journal Issue
1
Start Page
44
ISSN
0004-637X
1538-4357
Date Issued
2021-10-01
Author(s)
Abstract
We study the metallicity dependence of the H/H2 and C+/C/CO distributions in a self-regulated interstellar medium (ISM) across a broad range of metallicities (0.1 < Z/Z o˙ < 3). To this end, we conduct high-resolution (particle mass of 1 M o˙) hydrodynamical simulations coupled with a time-dependent H2 chemistry network. The results are then post-processed with an accurate chemistry network to model the associated C+/C/CO abundances, based on the time-dependent non-steady-state ("nonequilibrium") H2 abundances. We find that the time-averaged star formation rate and the ISM structure are insensitive to metallicity. The column densities relevant for molecular shielding appear correlated with the volume densities in gravitationally unstable gas. As metallicity decreases, H2 progressively deviates from steady state ("equilibrium") and shows shallow abundance profiles until they sharply truncate at the photodissociation fronts. In contrast, the CO profile is sharp and controlled by photodissociation as CO quickly reaches steady state. We construct effective 1D cloud models that successfully capture the time-averaged chemical distributions in simulations. At low metallicities, the steady-state model significantly overestimates the abundance of H2 in the diffuse medium. The overestimated H2, however, has little impact on CO. Consequently, the mass fraction of CO-dark H2 gas is significantly lower than what a fully steady-state model predicts. The mass ratios of H2/C+ and H2/C both show a weaker dependence on Z' than H2/CO, which potentially indicates that C+ and C could be alternative tracers for H2 at low Z' in terms of mass budget. Our chemistry code for post-processing is publicly available.
Publisher
American Astronomical Society
Type
journal article