The MOSDEF Survey: Direct-Method Metallicities and ISM Conditions at \(z\sim1.5-3.5\)

Abstract

We present detections of [OIII]\(\lambda\)4363 and direct-method metallicities for star-forming galaxies at \(z=1.7-3.6\). We combine new measurements from the MOSFIRE Deep Evolution Field (MOSDEF) survey with literature sources to construct a sample of 18 galaxies with direct-method metallicities at \(z>1\), spanning \(7.5<1\)2+log(O/H\()<8.2\) and log(M\(_*\)/M\(_{\odot})=7-10\). We find that strong-line calibrations based on local analogs of high-redshift galaxies reliably reproduce the metallicity of the \(z>1\) sample on average. We construct the first mass-metallicity relation at \(z>1\) based purely on direct-method O/H, finding a slope that is consistent with strong-line results. Direct-method O/H evolves by \(\lesssim0.1\) dex at fixed M\(_*\) and SFR from \(z\sim0-2.2\). We employ photoionization models to constrain the ionization parameter and ionizing spectrum in the high-redshift sample. Stellar models with super-solar O/Fe and binary evolution of massive stars are required to reproduce the observed strong-line ratios. We find that the \(z>1\) sample falls on the \(z\sim0\) relation between ionization parameter and O/H, suggesting no evolution of this relation from \(z\sim0\) to \(z\sim2\). These results suggest that the offset of the strong-line ratios of this sample from local excitation sequences is driven primarily by a harder ionizing spectrum at fixed nebular metallicity compared to what is typical at \(z\sim0\), naturally explained by super-solar O/Fe values at high redshift caused by rapid formation timescales. Given the extreme nature of our \(z>1\) sample, the implications for representative \(z\sim2\) galaxy samples at \(\sim10^{10}\) M\(_{\odot}\) are unclear, but similarities to \(z>6\) galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.