A Lyman-{\alpha} protocluster at redshift 6.9

TL;DR

This paper reports the discovery of a high-redshift protocluster at z=6.93, providing insights into early structure formation and the reionization process when the universe was less than 1 billion years old.

Contribution

It presents the first detailed study of a protocluster at this redshift, including its structure, galaxy composition, and implications for reionization.

Findings

Protocluster at z=6.93 with 21 Lyman-alpha galaxies

Contains two subprotoclusters likely to merge into a massive cluster

Ionized bubbles from member galaxies nearly fill the protocluster volume

Abstract

Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6. Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-$\alpha$ galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.