Line Overlap and Self-Shielding of Molecular Hydrogen in Galaxies

TL;DR

This paper investigates how line overlap in molecular hydrogen's Lyman and Werner bands enhances self-shielding in low-metallicity galaxies, affecting molecular hydrogen formation and star formation processes.

Contribution

The study introduces an empirical model incorporating line overlap effects into cosmological simulations, providing new fitting formulas for molecular hydrogen fractions under various conditions.

Findings

Line overlap significantly enhances H2 self-shielding in low-metallicity environments.

Line overlap has a minor impact on star formation compared to stellar feedback.

The model aligns well with observational data across different galactic environments.

Abstract

The effect of line overlap in the Lyman and Werner bands, often ignored in galactic studies of the atomic-to-molecular transition, greatly enhances molecular hydrogen self-shielding in low metallicity environments, and dominates over dust shielding for metallicities below about 10% solar. We implement that effect in cosmological hydrodynamics simulations with an empirical model, calibrated against the observational data, and provide fitting formulae for the molecular hydrogen fraction as a function of gas density on various spatial scales and in environments with varied dust abundance and interstellar radiation field. We find that line overlap, while important for detailed radiative transfer in the Lyman and Werner bands, has only a minor effect on star formation on galactic scales, which, to a much larger degree, is regulated by stellar feedback.