Kavli Affiliate: Debora Sijacki
| First 5 Authors: Kasper E. Heintz, Jake S. Bennett, Pascal A. Oesch, Albert Sneppen, Douglas Rennehan
| Summary:
Galaxy clusters are the most massive, gravitationally-bound structures in the
Universe, emerging through hierarchical structure formation of large-scale dark
matter and baryon overdensities. Early galaxy “proto-clusters” are believed
to be important physical drivers of the overall cosmic star-formation rate
density and serve as “hotspots” for the reionization of the intergalactic
medium. Our understanding of the formation of these structures at the earliest
cosmic epochs is, however, limited to sparse observations of their galaxy
members, or based on phenomenological models and cosmological simulations. Here
we report the detection of a massive neutral, atomic hydrogen (HI) gas
reservoir permeating a galaxy proto-cluster at redshift $z=5.4$, observed one
billion years after the Big Bang. The presence of this cold gas is revealed by
strong damped Lyman-$alpha$ absorption features observed in several background
galaxy spectra taken with JWST/NIRSpec in close on-sky projection. While
overall the sightlines probe a large range in HI column densities, $N_{rm HI}
= 10^{21.7}-10^{23.5}$ cm$^{-2}$, they are similar across nearby sightlines,
demonstrating that they probe the same dense, neutral gas. This observation of
a massive, large-scale overdensity of cold neutral gas challenges current
large-scale cosmological simulations and has strong implications for the
reionization topology of the Universe.
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