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Title: The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (zmean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass oflog(M*/M)med=8.290.43+0.51and a median star formation rate ofSFRHαmed=2.251.26+2.15Myr1. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of12+log(O/H)direct=7.880.22+0.25(0.150.06+0.12Z). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories(log(M*/M)med=8.920.22+0.31), we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at more » fixedM*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density ofne=10+215cm3(ne=10+74cm3) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

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Publication Date:
Journal Name:
The Astrophysical Journal
Page Range or eLocation-ID:
Article No. 108
DOI PREFIX: 10.3847
Sponsoring Org:
National Science Foundation
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