Clues to the origin of the mass-metallicity relation: Dependence on star formation rate and galaxy size

We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size ( as measured by the half-light radius, r(h)) on the mass-metallicity relation. We find that galaxies with high SSFR or large r(h) for their stellar mass have systematically lower gas-phase metallicities (by up to 0.2 dex) than galaxies with low SSFR or small r(h). We discuss possible origins for these dependencies, including galactic winds/ outflows, abundance gradients, environment, and star formation rate efficiencies.