Tracing the formation history of galaxy clusters into the epoch of reionization

TL;DR

This paper reviews the current understanding and future observational prospects of galaxy protoclusters from the epoch of reionization to lower redshifts, emphasizing their role in structure formation and galaxy evolution.

Contribution

It provides a comprehensive overview of how upcoming large telescopes will advance the study of protoclusters and their connection to galaxy and black hole formation.

Findings

Protoclusters are key to understanding galaxy cluster formation.

Upcoming telescopes will enable systematic studies of early universe structures.

Protoclusters may have significantly contributed to cosmic reionization.

Abstract

The large-scale overdensities of galaxies at z~2-7 known as protoclusters are believed to be the sites of cluster formation, and deep, wide survey projects such as the Large Synoptic Survey Telescope (LSST) and the Wide Field Infrared Survey Telescope (WFIRST) will deliver significant numbers of these interesting structures. Spectroscopic confirmation and interpretation of these targets, however, is still challenging, and will require wide-field multi-plexed spectroscopy on >20 m-class telescopes in the optical and near-infrared. In the coming decade, detailed studies of protoclusters will enable us, for the first time, to systematically connect these cluster progenitors in the early universe to their virialized counterparts at lower redshifts. This will allow us to address observationally the formation of brightest cluster galaxies and other cluster galaxy populations, the buildup of the intra-cluster light, the chemical enrichment history of the intra-cluster medium, and the formation and triggering of supermassive black holes in dense environments, all of which are currently almost exclusively approached either through the fossil record in clusters or through numerical simulations. Furthermore, at the highest redshifts (z~5-10), these large extended overdensities of star-forming galaxies are believed to have played an important role in the reionization of the universe, which needs to be tested by upcoming experiments. Theory and recent simulations also suggest important links between these overdensities and the formation of supermassive black holes, but observational evidence is still lacking. In this white paper we review our current understanding of this important phase of galaxy cluster history that will be explored by the next generation of large aperture ground-based telescopes GMT and TMT.