The Metallicity Dependence of the H_I Shielding Layers in Nearby Galaxies

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-H₂ 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.