The Direct-Method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High-Noon

Abstract

We present a Keck/MOSFIRE, rest-optical, composite spectrum of 16 typical, gravitationally-lensed, star-forming, dwarf galaxies at $1.7\lesssim z\lesssim 2.6$ ($z_{\mathrm{mean}}=2.30$), all chosen independent of emission-line strength. These galaxies have a median stellar mass of $\log (M_{\ast }/{\mathrm{M}}_{\odot }{)}_{\mathrm{med}}=8.29_{-0.43}^{+0.51}$ and a median star formation rate of $\mathrm{SF}{\mathrm{R}}_{\mathrm{H}\alpha }^{\mathrm{med}}=2.25_{-1.26}^{+2.15}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. We measure the faint, electron-temperature-sensitive, [O III] $\lambda$4363 emission line at $2.5\sigma$ ($4.1\sigma$) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of $12+\log (O/H{)}_{\mathrm{direct}}=7.88_{-0.22}^{+0.25}$ ($0.15_{-0.06}^{+0.12}{\mathrm{Z}}_{\odot }$). We investigate the applicability at high-$z$ of locally-calibrated, oxygen-based, strong-line metallicity relations, finding that the local reference calibrations of arXiv:1805.08224 best reproduce ($\lesssim 0.12$ dex) our composite metallicity at fixed strong-line ratio. At fixed $M_{\ast }$, our composite is well-represented by the $z\sim 2.3$ direct-method stellar mass$-$gas-phase metallicity relation (MZR) of arXiv:1907.00013. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed $M_{\ast }$ and SFR, of the locally-defined fundamental metallicity relation. We measure the doublet ratio [O II] $\lambda$3729/[O II] $\lambda 3726=1.56\pm 0.32$ ($1.51\pm 0.12$) and a corresponding electron density of $n_e=1_{-0}^{+215}\mathrm{c}{\mathrm{m}}^{-3}$ ($n_e=1_{-0}^{+74}\mathrm{c}{\mathrm{m}}^{-3}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at $z\sim 2$.

30 pages, 6 figures, 4 tables, abstract slightly abridged, use of PyNeb adopted (replacing use of IRAF) to calculate electron temperatures and densities; change in results is negligible, accepted for publication in ApJ

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