THE MOSDEF SURVEY: MASS, METALLICITY, AND STAR-FORMATION RATE AT z ∼ 2.3

We present results on the z 2.3 mass- metallicity relation ( MZR) using early observations from the MOSFIRE Deep Evolution Field survey. We use an initial sample of 87 star- forming galaxies with spectroscopic coverage of H beta, [ Oiii].5007, Ha, and [ N ii].6584 rest- frame optical emission lines, and estimate the gas- phase oxygen abundance based on the N2 and O3N2 strong- line indicators. We find a positive correlation between stellar mass and metallicity among individual z 2.3 galaxies using both the N2 and O3N2 indicators. We also measure the emission- line ratios and corresponding oxygen abundances for composite spectra in bins of stellar mass. Among composite spectra, we find a monotonic increase in metallicity with increasing stellar mass, offset 0.15- 0.3 dex below the local MZR. When the sample is divided at the median star- formation rate ( SFR), we do not observe significant SFR dependence of the z 2.3 MZR among either individual galaxies or composite spectra. We furthermore find that z 2.3 galaxies have metallicities 0.1 dex lower at a given stellar mass and SFR than is observed locally. This offset suggests that high- redshift galaxies do not fall on the local fundamental metallicity relation among stellar mass, metallicity, and SFR, and may provide evidence of a phase of galaxy growth in which the gas reservoir is built up due to inflow rates that are higher than star- formation and outflow rates. However, robust conclusions regarding the gas- phase oxygen abundances of high- redshift galaxies await a systematic reappraisal of the application of locally calibrated metallicity indicators at high redshift.