Lyα EMITTERS AT z = 7 IN THE SUBARU/XMM-NEWTON DEEP SURVEY FIELD: PHOTOMETRIC CANDIDATES AND LUMINOSITY FUNCTION

We conducted a deep narrowband NB973 (FWHM = 200 angstrom centered at 9755 angstrom) survey of z = 7 Ly alpha emitters (LAEs) in the Subaru/XMM-Newton Deep Survey Field, using the fully depleted CCDs newly installed on the Subaru Telescope Suprime-Cam, which is twice more sensitive to z = 7 Ly alpha at similar to 1 mu m than the previous CCDs. Reaching the depth 0.5 mag deeper than our previous survey in the Subaru Deep Field that led to the discovery of a z = 6.96 LAE, we detected three probable z = 7 LAE candidates. Even if all the candidates are real, the Lya luminosity function (LF) at z = 7 shows a significant deficit from the LF at z = 5.7 determined by previous surveys. The LAE number and Lya luminosity densities at z = 7 are similar to 7.7%-54% and similar to 5.5%-39%, respectively, of those at z = 5.7, to the Ly alpha line luminosity limit of L(Ly alpha) greater than or similar to 9.2 x 10(42) erg s(-1). This could be due to evolution of the LAE population at these epochs as a recent galaxy evolution model predicts that the LAE modestly evolves from z = 5.7 to 7. However, even after correcting for this effect of galaxy evolution on the decrease in LAE number density, the z = 7 Ly alpha LF still shows a deficit from z = 5.7 LF. This might reflect the attenuation of Ly alpha emission by neutral hydrogen remaining at the epoch of reionization and suggests that reionization of the universe might not be complete yet at z = 7. If we attribute the density deficit to reionization, the intergalactic medium transmission for Lya photons at z = 7 would be 0.4 <= T-Ly alpha(IGM) <= 1, supporting the possible higher neutral fraction at the earlier epochs at z > 6 suggested by the previous surveys of z = 5.7-7 LAEs, z similar to 6 quasars, and z > 6 gamma-ray bursts.