A Protocluster (PC) at the epoch of reionization (EoR) is an excellent laboratory to probe the earliest stage of galaxy/Universe evolution, such as the formation of massive galaxies and supermassive black holes (SMBHs), and large ionization bubbles. PCs are considered to accelerate the galaxy evolution and kickstart the cosmic reionization with an aid of efficient gas/galaxy accretion through filaments. We propose two-night Keck/DEIMOS observations of COSMOS2020-PCz6.05-01, an EoR PC candidate with 19 members found in the new COSMOS photo-z catalog. This proposed observation makes this overdensity the first EoR PC with >10 member galaxies with reliable stellar mass and star-formation (SF) rate thanks to the unique multi-band deep observations of the COSMOS field. Interestingly, the EoR PC has three passive candidates in the core, while such galaxies are not found in state-of-art cosmological simulations. The mechanism causing the SF quenching in EoR PCs is controversial and there are two scenarios: SMBH feedbacks and environmental effects. In order to examine the two scenarios, we (1) spectroscopically confirm the PC, (2) search for low-luminous quasars based on the Lyα luminosity and linewidth, and (3) identify the 3D structure around the PC.
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