The Detection of [O III]$\lambda$4363 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 $\sim 7.9$), star-forming (SFR $\sim$ 16 ${\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$), dwarf (log($M_{\ast }/M_{\odot }$) = $8.07-8.59$), Ly$\alpha$-emitter ($\mathrm{E}{\mathrm{W}}_0\sim 138$ A) 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 4.2$\sigma$ detection of the electron-temperature-sensitive [O III]$\lambda$4363 emission line and use this line to directly measure an oxygen abundance of 12+$\log$(O/H) = 8.06 $\pm$ 0.12 ($\sim 1/4{\mathrm{Z}}_{\odot }$). A1689-217 is the lowest mass galaxy at $z>$ 2 with an [O III]$\lambda$4363 detection. Using the rest-optical emission lines, we measure A1689-217's other nebular conditions including electron temperature ($T_e$([O III]) $\sim$ 14,000 K), electron density ($n_e\sim$ 220 $\mathrm{c}{\mathrm{m}}^{-3}$) and reddening ($E(B-V)\sim$ 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O III]$\lambda$4363 detections at $z\sim 0-3.1$, showing that the locally-calibrated, oxygen-based, strong-line relations are consistent from $z\sim 0-3.1$. We also show additional evidence that the ${\mathrm{O}}_{32}$ vs. ${\mathrm{R}}_{23}$ excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to $z\sim$ 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 $M_{\ast }$ and SFR, we find it to be consistent within the large scatter of the low-mass end of the Fundamental Metallicity Relation.

23 pages, 9 figures, 3 tables, 1 appendix - v2: Revised manuscript submitted to ApJ (added discussion/comments/refs), Rest-frame EW of H$\alpha$ and [O III]$\lambda$5007 corrected (results unaffected)