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

TL;DR

This paper develops an analytic model for the atomic to molecular transition in finite-sized interstellar clouds, improving understanding of molecular content in galaxies, especially in low-metallicity environments.

Contribution

It introduces a more realistic analytic solution for photodissociation regions in finite clouds, extending previous one-dimensional models to better match complex galactic environments.

Findings

Analytic expressions for molecular fraction with tens of percent accuracy.

Model compares well with numerical calculations in one-dimensional limit.

Clarifies the roles of self-shielding and dust shielding in H2 formation.

Abstract

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 clarify when self-shielding is the dominant process in H_2 formation, and under what circumstances shielding by dust makes a significant contribution.