The metallicity evolution of low mass galaxies: New constraints at
  intermediate redshift
  We present abundance measurements from 26 emission-line selected galaxies at
z~0.6-0.7. By reaching stellar masses as low as 10^8 M_{\sun}, these
observations provide the first measurement of the intermediate redshift
mass-metallicity (MZ) relation below 10^9 M_{\sun} For the portion of our
sample above M > 10^9 M_{\sun} (8/26 galaxies), we find good agreement with
previous measurements of the intermediate redshift MZ relation. Compared to the
local relation, we measure an evolution that corresponds to a 0.12 dex decrease
in oxygen abundances at intermediate redshifts. This result confirms the trend
that metallicity evolution becomes more significant towards lower stellar
masses, in keeping with a downsizing scenario where low mass galaxies evolve
onto the local MZ relation at later cosmic times. We show that these galaxies
follow the local fundamental metallicity relation, where objects with higher
specific (mass-normalized) star formation rates (SFRs) have lower
metallicities. Furthermore, we show that the galaxies in our sample lie on an
extrapolation of the SFR-M_{*} relation (the star-forming main sequence).
Leveraging the MZ relation and star-forming main sequence (and combining our
data with higher mass measurements from the literature), we test models that
assume an equilibrium between mass inflow, outflow and star formation. We find
that outflows are required to describe the data. By comparing different outflow
prescriptions, we show that momentum driven winds can describe the MZ relation;
however, this model under-predicts the amount of star formation in low mass
galaxies. This disagreement may indicate that preventive feedback from
gas-heating has been overestimated, or it may signify a more fundamental
deviation from the equilibrium assumption.
