LOW MASSES AND HIGH REDSHIFTS: THE EVOLUTION OF THE MASS-METALLICITY RELATION

Abstract

We present the first robust measurement of the high redshift mass-metallicity\n(MZ) relation at 10^{8}< M/M_{\\sun} < 10^{10}, obtained by stacking spectra of\n83 emission-line galaxies with secure redshifts between 1.3 < z < 2.3. For\nthese redshifts, infrared grism spectroscopy with the Hubble Space Telescope\nWide Field Camera 3 is sensitive to the R23 metallicity diagnostic:\n([OII]3726,3729 + [OIII] 4959,5007)/H\\beta. Using spectra stacked in four mass\nquartiles, we find a MZ relation that declines significantly with decreasing\nmass, extending from 12+log(O/H) = 8.8 at M=10^{9.8} M_{\\sun} to 12+log(O/H)=\n8.2 at M=10^{8.2} M_{\\sun}. After correcting for systematic offsets between\nmetallicity indicators, we compare our MZ relation to measurements from the\nstacked spectra of galaxies with M>10^{9.5} M_{\\sun} and z~2.3. Within the\nstatistical uncertainties, our MZ relation agrees with the z~2.3 result,\nparticularly since our somewhat higher metallicities (by around 0.1 dex) are\nqualitatively consistent with the lower mean redshift z=1.76 of our sample. For\nthe masses probed by our data, the MZ relation shows a steep slope which is\nsuggestive of feedback from energy-driven winds, and a cosmological downsizing\nevolution where high mass galaxies reach the local MZ relation at earlier\ntimes. In addition, we show that our sample falls on an extrapolation of the\nstar-forming main sequence (the SFR-M_{*} relation) at this redshift. This\nresult indicates that grism emission-line selected samples do not have\npreferentially high SFRs. Finally, we report no evidence for evolution of the\nmass-metallicity-SFR plane; our stack-averaged measurements show excellent\nagreement with the local relation.\n