Early Results from GLASS-JWST. XIV. A Spectroscopically Confirmed Protocluster 650 Million Years after the Big Bang

We present the spectroscopic confirmation of a protocluster at z = 7.88 behind the galaxy cluster Abell 2744 (hereafter A2744-z7p9OD). Using JWST NIRSpec, we find seven galaxies within a projected radius of 60 kpc. Although the galaxies reside in an overdensity around greater than or similar to 20x greater than a random volume, they do not show strong Ly alpha emission. We place 2 sigma upper limits on the rest-frame equivalent width alpha emission, we constrain the volume-averaged neutral fraction of hydrogen in the intergalactic medium to be x (HI) > 0.45 (68% C i). Using an empirical M (UV)-M (halo) relation for individual galaxies, we estimate that the total halo mass of the system is greater than or similar to 4 x 10(11) M (circle dot). Likewise, the line-of-sight velocity dispersion is estimated to be 1100 +/- 200 km s(-1). Using an empirical relation, we estimate the present-day halo mass of A2744-z7p9OD to be similar to 2 x 10(15) M (circle dot), comparable to the Coma cluster. A2744-z7p9OD is the highest redshift spectroscopically confirmed protocluster to date, demonstrating the power of JWST to investigate the connection between dark-matter halo assembly and galaxy formation at very early times with medium-deep observations at <20 hr total exposure time. Follow-up spectroscopy of the remaining photometric candidates of the overdensity will further refine the features of this system and help characterize the role of such overdensities in cosmic reionization.