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.


IPAC Authors


Takahiro Morishita

Assistant Scientist