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 (Multi-Object Spectrometer for InfraRed Exploration) and Keck/LRIS (Low Resolution Imaging Spectrometer) spectroscopy of A1689-217, a lensed (magnification similar to 7.9), star-forming (SFR similar to 16 M-circle dot yr(-1)), dwarf (log(M*/M-circle dot) = 8.07-8.59) Ly alpha emitter (EW0 similar to 138 angstrom) 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.+/-.0.12 (similar to 1/4 Z(circle dot)). 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]) similar to 14,000 K), electron density (n(e) similar to 220 cm(-3)), and reddening (E(B-V) similar to 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O III].4363 detections at z similar to 0-3.1, showing that the locally calibrated, oxygen-based, strong-line relations are consistent from z similar to 0 to 3.1. We also show additional evidence that the O-32 versus R-23 excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to z similar to 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 star formation rate, we find it to be consistent within the large scatter of the low-mass end of the fundamental metallicity relation.