2012ApJ...752..114S

We have performed deep imaging surveys for Lyα emitters (LAEs) at redshift ∼7.3 in two blank fields, the Subaru Deep Field (SDF) and the Subaru/XMM-Newton Deep survey Field (SXDF), using the Subaru/Suprime-Cam equipped with new red-sensitive CCDs and a new narrowband filter, NB1006 (λ_c= 10052 Å, FWHM Δλ = 214 Å). We identified four objects as LAE candidates that exhibit luminosity excess in NB1006. By carrying out deep follow-up spectroscopy for three of them using Subaru/FOCAS and Keck/DEIMOS, a definitively asymmetric emission line is detected for one of them, SXDF-NB1006-2. Assuming this line is Lyα, this object is an LAE at z = 7.215 which has a luminosity of 1.2^+1.5_–0.6x10⁴³ erg/s and a weighted skewness S_ω= 4.90±0.86. Another object, SDF-NB1006-2, shows variable photometry and is thus probably a quasar (QSO) or an active galactic nucleus. It shows an asymmetric emission line at 10076 Å which may be due to either Lyα at z = 7.288 or [O II] at z = 1.703. The third object, SDF-NB1006-1, is likely a galaxy with temporal luminosity enhancement associated with a supernova explosion, as the brightness of this object varies between the observed epochs. Its spectrum does not show any emission lines. The inferred decrease in the number density of LAEs toward higher redshift is n ^ z = 7.3Lyα_/nLyα_^ z = 5.7^ = 0.05^+0.11 _–0.05 from z = 5.7 to 7.3 down to L ^Lyα = 1.0x10⁴³ erg/s. The present result is consistent with the interpretation in previous studies that the neutral hydrogen fraction is rapidly increasing from z = 5.7 to 7.3.