A Characteristic Mass Scale in the Mass–Metallicity Relation of Galaxies

We study the shape of the gas-phase mass─metallicity relation (MZR) of a combined sample of present-day dwarf and high-mass star-forming galaxies using IZI, a Bayesian formalism for measuring chemical abundances presented in a previous publication. We observe a characteristic stellar mass scale at M * ≃ 109.5 M ☉, above which the inter-stellar medium undergoes a sharp increase in its level of chemical enrichment. In the 10⁶─109.5 M ☉ range the MZR follows a shallow power law (Z\∝ {M}*α) with slope α = 0.14 ± 0.08. Approaching M * ≃ 109.5 M ☉ the MZR steepens significantly, showing a slope of α = 0.37 ± 0.08 in the {10}9.5{--}{10}10.5 {M}ȯ range, and a flattening toward a constant metallicity at higher stellar masses. This behavior is qualitatively different from results in the literature that show a single power-law MZR toward the low-mass end. We thoroughly explore systematic uncertainties in our measurement, and show that the shape of the MZR is not induced by sample selection, aperture effects, a changing N/O abundance, the adopted methodology to construct the MZR, secondary dependences on star formation activity, or diffuse ionized gas contamination, but rather on differences in the method used to measure abundances. High-resolution hydrodynamical simulations of galaxies can qualitatively reproduce our result, and suggest a transition in the ability of galaxies to retain their metals for stellar masses above this threshold. The MZR characteristic mass scale also coincides with a transition in the scale height and clumpiness of cold gas disks, and a typical gas fraction below which the efficiency of star formation feedback for driving outflows is expected to decrease sharply.