We propose to use Keck MOSFIRE to carry out deep H and K-band spectroscopy of GN-z11 at z∼11.1. GN-z11 is the most distant galaxy known to date. It is surprisingly bright; theoretical models predicted that a search volume 10–50 times larger would have been necessary to find one such galaxy. Therefore, the discovery of GN-z11 sheds new light on galaxy formation in the early universe. However, the available information for this galaxy is very limited so far. In 2017A, we were awarded one Keck MOSFIRE night to observe GN-z11. From the resultant spectrum, we identified a pair of emission lines with 3−4σ significance at ∼23,500Å, likely corresponding to a CIII] λ1909Å doublet at z=11.314. The HST grism spectrum also shown an emission-like feature at ∼1.5µm, suggesting that GN-z11 is probably at z∼11.3, if that feature is Lyα. Therefore, an independent confirmation is urgently needed. The proposed observations will be critical to confirm the existence of the Lyα and CIII] lines in GN-z11, and put a strong constraint on the HeII λ1640Å flux. The detection of these lines will enable us to compute accurate redshift, calculate line flux, estimate ionizing state of the galaxy, and constrain Population III star formation rate in this unique galaxy.
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