Reionization and Galaxy Evolution Probed byz= 7 Lyα Emitters

Abstract

We have performed narrowband NB973 (bandwidth 200 Å centered at 9755 Å) imaging of the Subaru Deep Field (SDF) and found two z = 7 Lyα emitter (LAE) candidates down to NB973 = 24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z = 6.96 LAE. This has established a new redshift record, showing that galaxy formation was in progress just 750 Myr after the big bang. Meanwhile, the Lyα line luminosity function of LAEs is known to decline from z = 5.7 to 6.6 in the SDF; L^* at z = 6.6 is 40%-60% that at z = 5.7. We also confirm that the number density of z = 7 LAEs is only 17% of the density at z = 6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift could be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs and Lyman break galaxies, and a LAE evolution model based on hierarchical clustering, we find that galaxy evolution alone cannot entirely explain the decrease in density. This extra density deficit might reflect the attenuation of the Lyα photons from LAEs by the neutral hydrogen possibly left at the last stage of cosmic reionization at z ~ 6-7.