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 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 [O ii ] λλ 3726,3729, [Ne iii ] λ 3869, and H γ emission lines and a tentative detection of H δ , indicating z 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 α emission with a rest-frame equivalent width of EW(H α ) = 1200 Å. Based on the assumption of 0.5 Z ⊙ models and the Calzetti attenuation curve, GOODSN-17940 is characterized by . The Balmer decrement inferred from H α /H γ is used to dust correct the H α emission, yielding . These M * and star formation rate (SFR) values place GOODSN-17940 an order of magnitude in SFR above the z  ∼ 4 star-forming “main sequence.” Finally, we use the observed ratio of [Ne iii ]/[O ii ] to estimate the nebular oxygen abundance in GOODSN-17940, finding O/H ∼ 0.2 (O/H) ⊙ . Combining our new [Ne iii ]/[O ii ] measurement with those from stacked spectra at z  ∼ 0, 2, and 3, we show that GOODSN-17940 represents an extension to z  > 4 of the evolution toward higher [Ne iii ]/[O ii ] (i.e., lower O/H) at fixed stellar mass. It will be possible to perform the measurements presented here out to z  ∼ 10 using the James Webb Space Telescope .