The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey

We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000 star-forming galaxies at z~0.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses using new techniques that make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (+/-0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 10^8.5 to 10^10.5 M_solar h^-2₇₀, in good accord with known trends between luminosity and metallicity, but flattens above 10^10.5 M_solar. We use indirect estimates of the gas mass based on the Hα luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anticorrelated with baryonic mass, with low-mass dwarf galaxies being 5 times more metal depleted than L^* galaxies at z~0.1. Evidence for metal depletion is not confined to dwarf galaxies but is found in galaxies with masses as high as 10¹⁰ M_solar. We interpret this as strong evidence of both the ubiquity of galactic winds and their effectiveness in removing metals from galaxy potential wells.