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 deg^2 areas, respectively, selected from 2,230 (259) Lya
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 Lya 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 > 10^14 M_sun. We then investigate the correlation between LAE overdensity
delta and Lya rest-frame equivalent width EW_Lya^rest, because the cosmological
Lya RT model suggests that a slope of EW_Lya^rest-delta 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 Lya emission
from the partly neutral intergalactic medium (IGM). The available HSC data
suggest that the slope of the EW_Lya^rest-delta 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_Lya^rest - delta relation from z=5.7 to 7.3 by the upcoming
HSC observations providing large samples of LAEs at z=6.6-7.3.
