The MOSDEF Survey: First Measurement of Nebular Oxygen Abundance at $z>4$

Abstract

We present the first spectroscopic measurement of multiple rest-frame optical emission lines at $z>4$. During the MOSFIRE Deep Evolution Field (MOSDEF) survey, we observed the galaxy GOODSN-17940 with the Keck I/MOSFIRE spectrograph. The K-band spectrum of GOODSN-17940 includes significant detections of the [OII]$\lambda \lambda 3726,3729$, [NeIII]$\lambda 3869$, and H$\gamma$ emission lines and a tentative detection of H$\delta$, indicating $z_{\mathrm{spec}}=4.4121$. GOODSN-17940 is an actively star-forming $z>4$ galaxy based on its K-band spectrum and broadband spectral energy distribution. A significant excess relative to the surrounding continuum is present in the Spitzer/IRAC channel 1 photometry of GOODSN-17940, due primarily to strong H$\alpha$ emission with a rest-frame equivalent width of $\mbox{EW(H}α)=1200$ Å. Based on the assumption of $0.5Z_{\odot }$ models and the Calzetti attenuation curve, GOODSN-17940 is characterized by $M_{\ast }=5.0_{-0.2}^{+4.3}\times 10^9{M}_{\odot }$. The Balmer decrement inferred from H$\alpha$/H$\gamma$ is used to dust correct the H$\alpha$ emission, yielding $\mbox{SFR(H}α)=320^{+190}_{-140} M_{\odot}\mbox{ yr}^{-1}$. These $M_{\ast }$ and SFR values place GOODSN-17940 an order of magnitude in SFR above the $z\sim 4$ star-forming "main sequence." Finally, we use the observed ratio of [NeIII]/[OII] to estimate the nebular oxygen abundance in GOODSN-17940, finding $\mbox{O/H}\sim 0.2 \mbox{ (O/H)}_{\odot}$. Combining our new [NeIII]/[OII] measurement with those from stacked spectra at $z\sim 0, 2, \mbox{ and } 3$, we show that GOODSN-17940 represents an extension to $z>4$ of the evolution towards higher [NeIII]/[OII] (i.e., lower $\mbox{O/H}$) at fixed stellar mass. It will be possible to perform the measurements presented here out to $z\sim 10$ using the James Webb Space Telescope.

7 pages, 3 figures, accepted to ApJ Letters

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