Mass-dependent chemical enrichment sequences of SDSS star-forming galaxies out to z∼0.3 revealed by direct O & Ar abundances

Individual stars in the Milky Way (MW) and its satellites have been shown to trace galaxy stellar mass dependent sequences in the -abundance ([/Fe]) versus metallicity ([Fe/H]) plane. Testing the universality of such sequences has been elusive as deep absorption-line spectra required for [/Fe] and [Fe/H] measurements beyond the local group are mostly limited to integrated light from nearby, relatively high-mass, early-type galaxies. However, analogous to [/Fe] versus [Fe/H] for stars, we now have log(O/Ar) versus 12 + log(Ar/H) for the integrated nebular light of star-forming galaxies (SFGs). From Sloan-Digital Sky Survey observations of SFGs out to z, where we directly determined O & Ar abundances, we obtain for the first time the distribution of an ensemble of SFGs in the log(O/Ar) versus 12 + log(Ar/H) plane. We show that higher (M) and lower mass (M) SFGs clearly trace distinct mass dependent sequences in this plane. Such sequences are consistent with expectations from galaxy chemical evolution models of SFGs that are driven primarily by the interplay of core-collapse and Type Ia supernovae.