2023ApJ...947L..24M

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 ≳20× greater than a random volume, they do not show strong Lyα emission. We place 2σ upper limits on the rest-frame equivalent width <16-28 Å. Based on the tight upper limits to the Lyα 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 ≳4 × 10¹¹M_⊙. 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 ∼2 × 10¹⁵M_⊙, 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.