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

Creators: Gburek, Timothy; Siana, Brian; Alavi, Anahita; Emami, Najmeh; Richard, Johan; Freeman, William R.; Stark, Daniel P.; Snapp-Kolas, Christopher

Abstract

We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲ z ≲ 2.6 (z_(mean) = 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of log(M_*/M_⊙)_(med) = 8.29^(+0.51)_(-0.43) and a median star formation rate of SFR^(med)_(Hα) = 2.25^(+2.15)_(-1.26) M_⊙ yr⁻¹. We measure the faint electron-temperature-sensitive [O III] λ4363 emission line at 2.5σ (4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of 12 + log(O/H)_(direct) = 7.88^(+0.25)_(-0.22) (0.15^(+0.12)_(-0.06) Z_⊙). We investigate the applicability at high z of 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 fixed M_*, our composite is well represented by the z ∼ 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) = 8.92^(+0.31)_(-0.22), we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed M_* and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [O II] λ3729/[O II] λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of nₑ = 1⁺²¹⁵₋₀ cm⁻³ (nₑ = 1⁺⁷⁴₋₀ cm⁻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 ∼ 2.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programs #9289, #11710, #11802, #12201, #12931, #13389, #13498, #13504. This material is based upon work supported by the National Science Foundation under grant No. 1617013. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck:I (MOSFIRE) - , HST (WFC3 - , ACS) - .

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