The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies
  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.
