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We study the shape of the gas-phase mass-metallicity relation (MZR) of acombined sample of present-day dwarf and high-mass star-forming galaxies usingIZI, a Bayesian formalism for measuring chemical abundances presented in Blancet al. 2015. We observe a characteristic stellar mass scale at $M_* \simeq10^{9.5}$M$_{\odot}$, above which the ISM undergoes a sharp increase in itslevel of chemical enrichment. In the $10^{6}-10^{9.5}$M$_{\odot}$ range the MZRfollows a shallow power-law ($Z\propto M^{\alpha}_*$) with slope$\alpha=0.14\pm0.08$. At approaching $M_* \simeq 10^{9.5}$M$_{\odot}$ the MZRsteepens significantly, showing a slope of $\alpha=0.37\pm0.08$ in the$10^{9.5}-10^{10.5}$M$_{\odot}$ range, and a flattening towards a constantmetallicity at higher stellar masses. This behavior is qualitatively differentfrom results in the literature that show a single power-law MZR towards the lowmass end. We thoroughly explore systematic uncertainties in our measurement,and show that the shape of the MZR is not induced by sample selection, apertureeffects, a changing N/O abundance, the adopted methodology used to constructthe MZR, secondary dependencies on star formation activity, nor diffuse ionizedgas (DIG) contamination, but rather on differences in the method used tomeasure abundances. High resolution hydrodynamical simulations canqualitatively reproduce our result, and suggest a transition in the ability ofgalaxies to retain their metals for stellar masses above this threshold. TheMZR characteristic mass scale also coincides with a transition in the scaleheight and clumpiness of cold gas disks, and a typical gas fraction below whichthe efficiency of star formation feedback for driving outflows is expected todecrease sharply.

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