2010ApJ...722..803O

We conducted a deep narrowband NB973 (FWHM = 200 Å centered at 9755 Å) survey of z = 7 Lyα 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 Lyα at ∼1 µm 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 Lyα 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 Lyα luminosity densities at z = 7 are ∼7.7%-54% and ∼5.5%-39%, respectively, of those at z = 5.7, to the Lyα line luminosity limit of L(Lyα) ≳ 9.2x10⁴² erg/s. 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 Lyα LF still shows a deficit from z = 5.7 LF. This might reflect the attenuation of Lyα 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 Lyα photons at z = 7 would be 0.4 ≤ T ^IGM_Lyα≤ 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 ∼ 6 quasars, and z > 6 gamma-ray bursts.