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 (MOSDEF) survey. We use an
initial sample of 87 star-forming galaxies with spectroscopic coverage of
H\beta, [OIII]\lambda 5007, H\alpha, and [NII]\lambda 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.
