A Giant Protocluster of Galaxies at Redshift 5.7

TL;DR

This paper reports the discovery of a massive galaxy protocluster at redshift 5.7, providing insights into early universe structure formation and challenging current cosmological models.

Contribution

It presents the first identification of a giant protocluster at z=5.7 with detailed spectroscopic confirmation and analysis.

Findings

Contains at least 41 spectroscopically confirmed LAEs.

Occupies a volume of about 35x35x35 co-moving Mpc.

Will evolve into a galaxy cluster with mass ~3.6 x 10^{15} solar masses.

Abstract

Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology. Clusters with extended X-ray emission have been discovered at redshifts up to z ~ 2.5. Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshifts. 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 35x35x35 cubic co-moving megaparsecs. It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Lyman-alpha emitting galaxies (Lyman-alpha Emitters, or LAEs), including several previously reported LAEs. Its LAE density is 6.6 times the average density at z ~ 5.7. It is the only one of its kind in an 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) x 10^{15} solar masses, comparable to those of the most massive clusters or protoclusters known to date.