Silverrush. VII. Subaru/HSC identifications of 42 protocluster candidates AT z - 6 − 7 with the spectroscopic redshifts up to z = 6.574: Implications for cosmic reionization

We report fourteen and twenty-eight protocluster candidates at z = 5.7 and 6.6 over 14 and 19 deg² areas, respectively, selected from 2,230 (259) Lyα emitters (LAEs) photometrically (spectroscopically) identified with Subaru/Hyper Suprime-Cam (HSC) deep images (Keck, Subaru, and Magellan spectra and the literature data). Six out of the 42 protocluster candidates include 1 − 12 spectroscopically confirmed LAEs at redshifts up to z = 6.574. By the comparisons with the cosmological Lyα radiative transfer (RT) model reproducing LAEs with the reionization effects, we find that more than a half of these protocluster candidates are progenitors of the present-day clusters with a mass of & 10¹⁴M_☉. We then investigate the correlation between LAE overdensity δ and Lyα rest-frame equivalent width EW_Ly^rest_α, because the cosmological Lyα RT model suggests that a slope of EW_Ly^rest_α-δ relation is steepened towards the epoch of cosmic reionization (EoR), due to the existence of the ionized bubbles around galaxy overdensities easing the escape of Lyα emission from the partly neutral intergalactic medium (IGM). The available HSC data suggest that the slope of the EW_Ly^rest_α-δ correlation does not evolve from the post-reionization epoch z = 5.7 to the EoR z = 6.6 beyond the moderately large statistical errors. There is a possibility that we would detect the evolution of the EW_Ly^rest_α - δ relation from z = 5.7 to 7.3 by the upcoming HSC observations providing large samples of LAEs at z = 6.6 − 7.3.