The Detection of [O iii] λ4363 in a Lensed, Dwarf Galaxy at z = 2.59: Testing Metallicity Indicators and Scaling Relations at High Redshift and Low Mass*

Abstract

We present Keck/MOSFIRE and Keck/LRIS spectroscopy of A1689-217, a lensed (magnification ∼ 7.9), star-forming (SFR ∼ 16 M yr −1 ), dwarf (log(M * /M ) = 8.07−8.59) Lyα-emitter (EW 0 ∼ 138Å) at z = 2.5918. Dwarf galaxies similar to A1689-217 are common at high redshift and likely responsible for reionization, yet few have been studied with detailed spectroscopy. We report a 4.2σ detection of the electron-temperature-sensitive [O III]λ4363 emission line, the second such detection at z > 2, and use this line to directly measure an oxygen abundance of 12+log(O/H) = 8.06 ± 0.12 (∼ 1/4 Z ). Using the rest-optical emission lines, we measure A1689-217's other nebular conditions including electron temperature (T e ([O III]) ∼ 14,000 K), electron density (n e ∼ 220 cm −3 ) and reddening (E(B −V ) ∼ 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O III]λ4363 detections at z ∼ 0 − 3.1, showing that the locally-calibrated, oxygenbased, strong-line relations are consistent from z ∼ 0 − 3.1. We also show additional evidence that the O 32 vs. R 23 excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to z ∼ 3.1. From this excitation diagram and the strong-line ratio − metallicity plots, we observe that the ionization parameter at fixed O/H is consistent with no redshift evolution. Although A1689-217 is metal-rich for its M * and SFR, we find it to be consistent within the large scatter of the low-mass end of the Fundamental Metallicity Relation.

The Mass–Metallicity Relation at z ∼ 1–2 and Its Dependence on the Star Formation Rate

“…Overall, the agreement between the latest photoionization models and nearby H II regions suggests a convergence of oxygen abundance indicators, with systematic uncertainties greatly reduced from the 0.7 dex reported by Kewley & Ellison (2008). Further supporting this conclusion, recent detections of [O III] λ4363 in high-redshift galaxies confirm that low-redshift empirical strong-line calibrations agree with direct-method based measurements, within the uncertainties (Jones et al 2015a;Gburek et al 2019;Sanders et al 2020). These findings suggest that empirically derived strong-line calibrations, tied to direct-method metallicities, are applicable at high redshifts.…”

The mass-metallicity relation at z~1-2 and its dependence on star formation rate

“…Overall, the agreement between the latest photoionization models and nearby H II regions suggests a convergence of oxygen abundance indicators, with systematic uncertainties greatly reduced from the 0.7 dex reported by Kewley & Ellison (2008). Further supporting this conclusion, recent detections of [O III] λ4363 in high redshift galaxies confirm that low redshift empirical strong line calibrations agree with direct-method based measurements, within the uncertainties (Jones et al 2015a;Gburek et al 2019;Sanders et al 2020). These findings suggests that empirically derived strongline calibrations, tied to direct-method metallicities are applicable at high redshfifts.…”

The Ionizing Photon Production Efficiency (ξion) of Lensed Dwarf Galaxies at z ∼ 2*

“…The final spectroscopic redshift of a galaxy was determined via the weighted average of the redshifts fit to the different filters. More details about the spectroscopic line measurements of [O II] and other, fainter optical lines not considered here can be found in Gburek et al (2019). To assess the quality of the fits to the spectra, posterior histograms were output for each free parameter (as well as for the fluxes), and 68% confidence intervals were fit to the histograms.…”