The First Systematic Survey for Lyman Alpha Emitters at z=7.3 with
  Red-sensitive Subaru/Suprime-Cam
  We have performed deep imaging surveys for LyA 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 narrow-band filter, NB1006 (lambda_c=10052 Ang,
FWHM=214 Ang). 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
LyA, this object is a LAE at z=7.215 which has luminosity of 1.2^{+1.5}_{-0.6}
x 10^43 [erg s-1] and a weighted skewness S_w=4.90+-0.86. Another object,
SDF-NB1006-2, shows variable photometry and is thus probably a quasar (QSO) or
an active galactic nucleus (AGN). It shows an asymmetric emission line at 10076
Ang, which may be due to either LyA at z=7.288 or [OII] 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_LyA(z=7.3)/n_LyA(z=5.7) = 0.05^+0.11_-0.05 from z=5.7 to 7.3 down to
L(LyA)=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.
