Abstract

We present the first robust measurement of the high redshift mass-metallicity (MZ) relation at 10^8< M/M_\sun < 10^10, obtained by stacking spectra of 83 emission-line galaxies with secure redshifts between 1.3 < z < 2.3. For these redshifts, infrared grism spectroscopy with the Hubble Space Telescope Wide Field Camera 3 is sensitive to the R23 metallicity diagnostic: (OII3726,3729 + OIII 4959,5007)/H\beta. Using spectra stacked in four mass quartiles, we find a MZ relation that declines significantly with decreasing mass, extending from 12+log(O/H) = 8.8 at M=10^9.8 M_\sun to 12+log(O/H)= 8.2 at M=10^8.2 M_\sun. After correcting for systematic offsets between metallicity indicators, we compare our MZ relation to measurements from the stacked spectra of galaxies with M>10^9.5 M_\sun and z~2.3. Within the statistical uncertainties, our MZ relation agrees with the z~2.3 result, particularly since our somewhat higher metallicities (by around 0.1 dex) are qualitatively consistent with the lower mean redshift z=1.76 of our sample. For the masses probed by our data, the MZ relation shows a steep slope which is suggestive of feedback from energy-driven winds, and a cosmological downsizing evolution where high mass galaxies reach the local MZ relation at earlier times. In addition, we show that our sample falls on an extrapolation of the star-forming main sequence (the SFR-M_* relation) at this redshift. This result indicates that grism emission-line selected samples do not have preferentially high SFRs. Finally, we report no evidence for evolution of the mass-metallicity-SFR plane; our stack-averaged measurements show excellent agreement with the local relation.

Description

[1309.4458] Low Masses and High Redshifts: The Evolution of the Mass-Metallicity Relation

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