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

April 2023 • 2023ApJ...947L..24M

Authors • Morishita, Takahiro • Roberts-Borsani, Guido • Treu, Tommaso • Brammer, Gabriel • Mason, Charlotte A. • Trenti, Michele • Vulcani, Benedetta • Wang, Xin • Acebron, Ana • Bahé, Yannick • Bergamini, Pietro • Boyett, Kristan • Bradac, Marusa • Calabrò, Antonello • Castellano, Marco • Chen, Wenlei • De Lucia, Gabriella • Filippenko, Alexei V. • Fontana, Adriano • Glazebrook, Karl • Grillo, Claudio • Henry, Alaina • Jones, Tucker • Kelly, Patrick L. • Koekemoer, Anton M. • Leethochawalit, Nicha • Lu, Ting-Yi • Marchesini, Danilo • Mascia, Sara • Mercurio, Amata • Merlin, Emiliano • Metha, Benjamin • Nanayakkara, Themiya • Nonino, Mario • Paris, Diego • Pentericci, Laura • Rosati, Piero • Santini, Paola • Strait, Victoria • Vanzella, Eros • Windhorst, Rogier A. • Xie, Lizhi

Abstract • 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 × 10¹¹ M _⊙. 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 × 10¹⁵ M _⊙, 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.

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