THE FIRST SYSTEMATIC SURVEY FOR Lyα EMITTERS AT z=7.3 WITH RED-SENSITIVE SUBARU/SUPRIME-CAM

We have performed deep imaging surveys for Ly alpha emitters (LAEs) at redshift similar to 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 (lambda(c) = 10052 angstrom, FWHM Delta lambda = 214 angstrom). 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 Ly alpha, this object is an LAE at z = 7.215 which has a luminosity of 1.2(-0.6)(+1.5) x 10(43) erg s(-1) and a weighted skewness S-omega = 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 angstrom which may be due to either Ly alpha 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(Ly alpha)(z-7.3)/n(Ly alpha)(z-5.7) = 0.05(-0.05)(+0.11) from z = 5.7 to 7.3 down to L-Ly alpha = 1.0 x 10(43) erg s(-1). 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.