2017ApJ...844...85O

We detect 20 z = 7.0 Lyα emitter (LAE) candidates to L(Lyα) ∼ 2 x 10⁴² erg s^–1 or 0.3 L_z=7^* and in a volume of 6.1 x 10⁵ Mpc³ in the Subaru Deep Field and the Subaru/XMM-Newton Deep Survey field by 82 hr and 37 hr of Subaru Suprime-Cam narrowband NB973 and reddest optical y-band imaging. We compare their Lyα and UV luminosity functions (LFs) and densities and Lyα equivalent widths (EWs) to those of z = 5.7, 6.6, and 7.3 LAEs from previous Suprime-Cam surveys. The Lyα LF (density) rapidly declines by a factor of x1.5 (1.9) in L(Lyα) at z = 5.7-6.6 (160 Myr), x1.5 (1.6) at z = 6.6-7.0 (60 Myr) at the faint end, and x2.0 (3.8) at z = 7.0-7.3 (40 Myr). Also, in addition to the systematic decrease in EW at z = 5.7-6.6 previously found, two-thirds of the z = 7.0 LAEs detected in the UV continuum exhibit lower EWs than the z = 6.6 ones. Moreover, while the UV LF and density do not evolve at z = 5.7-6.6, they modestly decline at z = 6.6-7.0, implying galaxy evolution contributing to the decline of the Lyα LF. Comparison of the z = 7.0 Lyα LF to the one predicted by an LAE evolution model further reveals that galaxy evolution alone cannot explain all of the decline of the Lyα LF. If we attribute the discrepancy to Lyα attenuation by neutral hydrogen, the intergalactic medium transmission of Lyα photons at z = 7.0 would be T_Lyα^IGM 0.6–0.7. It is lower (higher) than the T_Lyα^IGM at z = 6.6 (7.3) derived by previous studies, suggesting rapid increase in neutral fraction at z > 6.