Ultra-deep Keck/MOSFIRE Spectroscopic Observations of z ∼ 2 Galaxies: Direct Oxygen Abundances and Nebular Excitation Properties

Using deep near-infrared Keck/MOSFIRE observations, we analyze the rest-optical spectra of eight star-forming galaxies in the COSMOS and GOODS-N fields. We reach integration times of ∼10 hr in the deepest bands, pushing the limits on current ground-based observational capabilities. The targets fall into two redshift bins, of five galaxies at z ∼ 1.7 and three galaxies at z ∼ 2.5, and were selected as likely to yield significant auroral-line detections. Even with long integration times, detection of the auroral lines remains challenging. We stack the spectra together into subsets based on redshift, improving the signal-to-noise ratio on the [O iii]λ4364 auroral emission line and, in turn, enabling a direct measurement of the oxygen abundance for each stack. We compare these measurements to commonly employed strong-line ratios alongside measurements from the literature. We find that the stacks fall within the distribution of z > 1 literature measurements, but a larger sample size is needed to robustly constrain the relationships between strong-line ratios and oxygen abundance at high redshift. We additionally report detections of [O i]λ6302 for nine individual galaxies and composite spectra of 21 targets in the MOSFIRE pointings. We plot their line ratios on the [O iii]λ5008/Hβ versus [O i]λ6302/Hα diagnostic diagram, comparing our targets to local galaxies and H ii regions. We find that the [O i]/Hα ratios in our sample of galaxies are consistent with being produced in gas ionized by α-enhanced massive stars, as has been previously inferred for rapidly forming galaxies at early cosmic times.