The GALEX Arecibo SDSS Survey V: The Relation between the HI Content of
  Galaxies and Metal Enrichment at their Outskirts
  We have obtained long-slit spectra of 174 star-forming galaxies with stellar
masses greater than 10^10 M_\odot from the GALEX Arecibo SDSS (GASS) survey.
These galaxies have both HI and H_2 mass measurements. The average metallicity
profile is strikingly flat out to R_90, the radius enclosing 90% of the r-band
light. Metallicity profiles which decline steadily with radius are found
primarily for galaxies in our sample with low stellar mass (Log(M_*)<10.2),
concentration, and/or mean stellar mass density. Beyond ~R_90, however, around
10 percent of the galaxies in our sample exhibit a sharp downturn in
metallicity. Remarkably, we find that the magnitude of the outer metallicity
drop is well correlated with the total HI content of the galaxy (measured as
f_HI=M_HI/M_*). We examine the radial profiles of stellar population ages and
star formation rate densities, and conclude that the galaxies with largest
outer metallicity drops are actively growing their stellar disks, with mass
doubling times across the whole disk only one third as long as a typical GASS
galaxy. We also describe a correlation between local stellar mass density and
metallicity, which is valid across all galaxies in our sample. We argue that
much of the recent stellar mass growth at the edges of these galaxies can be
linked to the accretion or radial transport of relatively pristine gas from
beyond the galaxies' stellar disks.
