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We have obtained long-slit spectra of 174 star-forming galaxies with stellarmasses 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 metallicityprofile is strikingly flat out to R_90, the radius enclosing 90% of the r-bandlight. Metallicity profiles which decline steadily with radius are foundprimarily for galaxies in our sample with low stellar mass (Log(M_*)<10.2),concentration, and/or mean stellar mass density. Beyond ~R_90, however, around10 percent of the galaxies in our sample exhibit a sharp downturn inmetallicity. Remarkably, we find that the magnitude of the outer metallicitydrop is well correlated with the total HI content of the galaxy (measured asf_HI=M_HI/M_*). We examine the radial profiles of stellar population ages andstar formation rate densities, and conclude that the galaxies with largestouter metallicity drops are actively growing their stellar disks, with massdoubling times across the whole disk only one third as long as a typical GASSgalaxy. We also describe a correlation between local stellar mass density andmetallicity, which is valid across all galaxies in our sample. We argue thatmuch of the recent stellar mass growth at the edges of these galaxies can belinked to the accretion or radial transport of relatively pristine gas frombeyond the galaxies' stellar disks.

The GALEX Arecibo SDSS Survey V: The Relation between the HI Content of Galaxies and Metal Enrichment at their Outskirts