Reionization and the ISM/Stellar Origins with JWST and ALMA (RIOJA): The Core of the Highest-redshift Galaxy Overdensity at z=7.88 Confirmed by NIRSpec/JWST

The protoclusters in the epoch of reionization, traced by galaxy overdensity regions, are ideal laboratories for studying the process of stellar assembly and cosmic reionization. We present the spectroscopic confirmation of the core of the most distant protocluster at z = 7.88, A2744-z7p9OD, with the James Webb Space Telescope NIRSpec integral field unit spectroscopy. The core region includes as many as four galaxies detected in [O III] 4960 and 5008 angstrom in a small area of similar to 3 '' x 3 '', corresponding to similar to 11 x 11 kpc, after the lensing magnification correction. Three member galaxies are also tentatively detected in dust continuum in Atacama Large Millimeter/submillimeter Array Band 6, which is consistent with their red ultraviolet continuum slopes, beta similar to -1.3. The member galaxies have stellar masses in the range of log(M-*/M-circle dot) similar to 7.6-9.2 and star formation rates of similar to 3-50 M-circle dot yr(-1), showing a diversity in their properties. FirstLight cosmological simulations reproduce the physical properties of the member galaxies including the stellar mass, [O III] luminosity, and dust-to-stellar mass ratio, and predict that the member galaxies are on the verge of merging in a few to several tens of Myr to become a large galaxy with M-* similar to 6 x 10(9)M(circle dot). The presence of a multiple merger and evolved galaxies in the core region of A2744-z7p9OD indicates that environmental effects are already at work 650 Myr after the Big Bang.