2018MNRAS.481.1690C

The gas-phase metallicity of low-mass galaxies increases with increasing stellar mass (M_*) and is nearly constant for high-mass galaxies. Theory suggests that this tight mass-metallicity relationship is shaped by galactic outflows removing metal-enriched gas from galaxies. Here, we observationally model the outflow metallicities of the warm outflowing phase from a sample of seven local star-forming galaxies with stellar masses between 10⁷and 10¹¹ M_☉. We estimate the outflow metallicities using four weak rest-frame ultraviolet absorption lines, the observed stellar continua, and photoionization models. The outflow metallicity is flat with M_*, with a median metallicity of 1.0 ± 0.6 Z_☉. The observed outflows are metal-enriched: low and high-mass galaxies have outflow metallicities 10-50 and 2.6 times larger than their interstellar medium (ISM) metallicities, respectively. The observed outflows are mainly composed of entrained ISM gas with at most 22 per cent of the metals directly coming from recent supernovae enrichment. The metal outflow rate shallowly increases with M_*, as M_*^0.2 ±0.1^ because the mass outflow rate shallow increases with M_*. Finally, we normalize the metal outflow rate by the rate at which star formation retains metals to calculate the metal-loading factor. The metal-loading factor inversely scales with M_*. The normalization and scaling of the metal-loading factor agree with analytic expressions that reproduce observed mass-metallicity relations. Galactic outflows fundamentally shape the observed mass-metallicity relationship.