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 [ O ii ] \lambda \lambda 3726 , 3729 , [ Ne iii ] \lambda 3869 , and H \gamma emission lines and a tentative detection of H \delta , 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 \alpha emission with a rest-frame equivalent width of \mbox { EW ( H } \alpha ) = 1200 Å .
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 [ Ne iii ] / [ O ii ] to estimate the nebular oxygen abundance in GOODSN-17940 , finding \mbox { O / H } \sim 0.2 \mbox { ( O / H ) } _ { \odot } .
Combining our new [ Ne iii ] / [ O ii ] 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 [ 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 \sim 10 using the James Webb Space Telescope .