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arXiv · 1707.05331

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

Alice E. Shapley·Ryan L. Sanders·Naveen A. Reddy·Mariska Kriek·William R. Freeman·Bahram Mobasher·Brian Siana·Alison L. Coil·Gene C. K. Leung·Laura deGroot·Irene Shivaei·Sedona H. Price·Mojegan Azadi·James Aird
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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]$\lambda3869$, and H$\gamma$ emission lines and a tentative detection of H$\delta$, indicating $z_{\rm{spec}}=4.4121$. 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 $\mbox{EW(H}\alpha)=1200$ \AA. Based on the assumption of $0.5 Z_{\odot}$ models and the Calzetti attenuation curve, GOODSN-17940 is characterized by $M_*=5.0^{+4.3}_{-0.2}\times 10^9 M_{\odot}$. The Balmer decrement inferred from H$\alpha$/H$\gamma$ is used to dust correct the H$\alpha$ emission, yielding $\mbox{SFR(H}\alpha)=320^{+190}_{-140} M_{\odot}\mbox{ yr}^{-1}$. These $M_*$ 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.

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Alice E. Shapley, Ryan L. Sanders, Naveen A. Reddy, Mariska Kriek, William R. Freeman, Bahram Mobasher, Brian Siana, Alison L. Coil, Gene C. K. Leung, Laura deGroot, Irene Shivaei, Sedona H. Price, Mojegan Azadi, James Aird. 2017-07-17. The MOSDEF Survey: First Measurement of Nebular Oxygen Abundance at $z>4$. https://doi.org/10.3847/2041-8213/aa8815

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