http://arxiv.org/abs/2211.09097
We present the spectroscopic confirmation of a protocluster at z = 7.89 behind the galaxy cluster Abell 2744. Using JWST NIRSpec, we find six galaxies within a projected distance of 60kpc, accounting for lensing magnification. We characterize the physical properties of the galaxies via SED fitting of deep HST+JWST imaging data covering 0.4-5.0$\mu$m. Although the galaxies reside in an overdensity around $\sim130$x 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 $<$16-26AA. Based on the tight upper limits to the Ly$\alpha$ emission, we constrain the neutral fraction of hydrogen along this line of sight to be $x_\mathrm{HI} > 0.45$ (68 % CI). Using an empirical $M_\mathrm{UV}$-$M_\mathrm{halo}$ relation for individual galaxies, we estimate that the total halo mass of the system is $> 4$x$10^{11} M_\odot$. Likewise, the line of sight velocity dispersion is estimated to be $1200 \pm 300$ km/s. This 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 hrs 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.
T. Morishita, G. Roberts-Borsani, T. Treu, et. al.
Thu, 17 Nov 22
15/63
Comments: Submitted to ApJL. 11 pages. 4 figures. 2 tables
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