THE METAL-ENRICHED OUTER DISK OF NGC 2915

We present optical emission-line spectra for outlying H II regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer disk of NGC 2915 to be enriched to a metallicity of 0.4 Zsun. An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-forming core is similarly underabundant for its gas fraction. Star formation rates derived from very deep ∼14 ks GALEX FUV exposures indicate that the low level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider three plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.