Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology1,2. Clusters with extended X-ray emission have been discovered at redshifts up to z ≈ 2.53-7. Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshifts8-14. It is, however, very challenging to find the largest protoclusters at early times when they start to assemble. Here we report a giant protocluster of galaxies at redshift z ≈ 5.7, when the Universe was only one billion years old. This protocluster occupies a volume of about 353 cubic co-moving megaparsecs (cMpc3). It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Lyα-emitting galaxies (Lyα Emitters, or LAEs), including several previously reported LAEs9. Its LAE density is 6.6 times the average density at z ≈ 5.7. It is the only one of its kind in a LAE survey in four square degrees on the sky. Such a large structure is also rarely seen in current cosmological simulations. This protocluster will collapse into a galaxy cluster with a mass of (3.6 ± 0.9) × 1015 solar masses (M☉), comparable to those of the most massive clusters or protoclusters known to date.
|Original language||English (US)|
|State||Published - Oct 12 2018|
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