The Metallicity Dependence of the H i Shielding Layers in Nearby Galaxies

Abstract

We investigate the metallicity dependence of H i surface densities in star-forming regions along many lines of sight within 70 nearby galaxies, probing kiloparsec to 50 pc scales. We employ H i , 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 H i surface densities saturate at sufficiently large total gas surface density. The maximal H i 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 H i gas is depressed by factors of ∼ 2. The observed behavior is naturally reproduced by metallicity dependent shielding theories for the H i -to-H 2 transitions in star-forming galaxies. We show that the inverse scaling of the maximal H i columns with metallicity suggests that the area filling fraction of atomic-molecular complexes in galaxies is of the order of unity, and weakly dependent on metallicity.