The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies

TL;DR

This study examines how metallicity influences HI surface densities in star-forming regions across 70 nearby galaxies, revealing a saturation behavior and inverse relation with metallicity consistent with shielding theories.

Contribution

It provides the first extensive observational analysis of metallicity dependence of HI shielding layers across diverse galaxy environments and scales.

Findings

HI surface densities saturate at high gas densities at intermediate to high metallicities.

Maximal HI columns are inversely proportional to metallicity.

The area filling fraction of atomic-molecular complexes is near unity and weakly dependent on metallicity.

Abstract

We investigate the metallicity dependence of HI surface densities in star-forming regions along many lines of sight within 70 nearby galaxies, probing kpc to 50 pc scales. We employ HI, SFR, stellar mass, and metallicity (gradient) measurements from the literature, spanning a wide range (5 dex) in stellar and gas mass and (1.6 dex) in metallicity. We consider metallicities as observed, or rescaled to match the mass-metallicity relation determined for SDSS galaxies. At intermediate to high metallicities (0.3-2 times solar), we find that the HI surface densities saturate at sufficiently large total gas surface density. The maximal HI columns vary approximately inversely with metallicity, and show little variation with spatial resolution, galactocentric radius, or among galaxies. In the central parts of massive spiral galaxies the HI gas is depressed by factors of 2. The observed behavior is naturally reproduced by metallicity dependent shielding theories for the HI-to-H2 transitions in star-forming galaxies. We show that the inverse scaling of the maximal HI columns with metallicity suggests that the area filling fraction of atomic-molecular complexes in galaxies is of order unity, and weakly dependent on metallicity.