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.
|Journal||Astrophysical Journal Letters|
|State||Published - Sep 10 2017|
Bibliographical noteFunding Information:
We acknowledge support from NSF AAG grants AST-1312780, 1312547, 1312764, and 1313171; grant AR-13907 from the Space Telescope Science Institute; and grant NNX16AF54G from the NASA ADAP program. J.A. acknowledges support from ERC Advanced Grant FEEDBACK 340442. We also acknowledge the 3D-HST collaboration, who provided us with spectroscopic and photometric catalogs used to select MOSDEF targets and derive stellar population parameters. We thank Daniel Stark for insightful comments. We wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests.
* 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.
© 2017. The American Astronomical Society. All rights reserved.
- galaxies: evolution
- galaxies: high-redshift
- galaxies: ISM
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science