2019ApJ...877...51C

We present evidence for the discovery of a protocluster of starburst galaxies (Lyα emitters, or LAEs) near the end of the epoch of reionization. The recent trend in the search for high-redshift protoclusters focuses on utilizing bias tracers, such as luminous starburst galaxies, as signposts of overdensities. Thus, we conducted a photometric selection of LAE candidates around a pair of spatially close, luminous LAEs at z = 6.5 in the Subaru/XMM-Newton Deep Survey field, using OSIRIS in its imaging mode at the 10.4 m Gran Telescopio Canarias in La Palma, Spain. The spectroscopic follow-up was done with OSIRIS in its multiobject spectroscopy capability. We have spectroscopically confirmed 10 LAEs via their recognizable Lyα emission feature. The redshifts of these LAEs shed light on their 3D distributions within the observing window defined by the photometric selection. We have derived the galaxy number density contrast of δ_gal=3.18_–1.99^+3.47, which led to the expected mass of the overdensity of 8.40_–1.39^+2.98×10¹⁴M_☉. We also found evidence for the presence of a virialized core with M₂₀₀=4.06_–1.90^+2.77×10¹³M_☉ within this overdensity. Based on the extended Press-Schechter formalism, this overdensity would continue to grow in the linear regime and collapse to form a galaxy cluster at z_coll=0.84_–0.43^+0.57. By the time this protocluster reaches z = 0, it will be a massive cluster of galaxies with mass 1.54_–0.69^+1.12×10¹⁵M_☉, comparable to the Coma cluster. Thus, our careful analysis has pointed to evidence that this protocluster would evolve into a Coma-analog cluster in the present-day universe.