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

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]λλ3726,3729, [Ne III]λ3869, and Hγ emission lines and a tentative detection of Hδ, indicating zspec = 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 Hα emission with a rest-frame equivalent width of EW(Hα) = 1200 Å. Based on the assumption of 0.5 Ze models and the Calzetti attenuation curve, GOODSN-17940 is characterized by M* = 5.0 ^ +4.3, -0.2 x 10^9 M⊙ The Balmer decrement inferred from Hα/Hγ is used to dust correct the Hα emission, yielding SFR (Hα) = 320 ^ +190, -140 M⊙ yr^-1.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)e. 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