The MOSDEF Survey: First Measurement of Nebular Oxygen Abundance at z > 4* * Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W.M. Keck Foundation.
Published Web Location
https://iopscience.iop.org/article/10.3847/2041-8213/aa8815/metaAbstract
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 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 Hα emission with a rest-frame equivalent width of EW(Hα) = 1200. Based on the assumption of 0.5 Z o models and the Calzetti attenuation curve, GOODSN-17940 is characterized by . The Balmer decrement inferred from Hα/Hγ is used to dust correct the Hα 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)o. 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.
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