Molecular cooling in the diffuse interstellar medium

TL;DR

This study models the thermal evolution of diffuse interstellar gas, showing that molecular hydrogen becomes a key coolant at low metallicities when metal line cooling diminishes, especially under weak radiation fields.

Contribution

It demonstrates that H2 cooling dominates in low-metallicity, low-density interstellar medium, providing new insights into thermal processes in such environments.

Findings

H2 cooling is negligible at solar metallicity.

H2 becomes dominant below 0.1 Z_solar metallicity.

H2 cooling depends on the ratio of radiation field to gas density.

Abstract

We use a simple one-zone model of the thermal and chemical evolution of interstellar gas to study whether molecular hydrogen (H2) is ever an important coolant of the warm, diffuse interstellar medium (ISM). We demonstrate that at solar metallicity, H2 cooling is unimportant and the thermal evolution of the ISM is dominated by metal line cooling. At metallicities below 0.1 Z_solar, however, metal line cooling of low density gas quickly becomes unimportant and H2 can become the dominant coolant, even though its abundance in the gas remains small. We investigate the conditions required in order for H2 to dominate, and show that it provides significant cooling only when the ratio of the interstellar radiation field strength to the gas density is small. Finally, we demonstrate that our results are insensitive to changes in the initial fractional ionization of the gas or to uncertainties in the nature of the dust present in the low-metallicity ISM.