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The realm of the galaxy protoclusters

A review

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Abstract

The study of galaxy protoclusters is beginning to fill in unknown details of the important phase of the assembly of clusters and cluster galaxies. This review describes the current status of this field and highlights promising recent findings related to galaxy formation in the densest regions of the early universe. We discuss the main search techniques and the characteristic properties of protoclusters in observations and simulations, and show that protoclusters will have present-day masses similar to galaxy clusters when fully collapsed. We discuss the physical properties of galaxies in protoclusters, including (proto-)brightest cluster galaxies, and the forming red sequence. We highlight the fact that the most massive halos at high redshift are found in protoclusters, making these objects uniquely suited for testing important recent models of galaxy formation. We show that galaxies in protoclusters should be among the first galaxies at high redshift making the transition from a gas cooling regime dominated by cold streams to a regime dominated by hot intracluster gas, which could be tested observationally. We also discuss the possible connections between protoclusters and radio galaxies, quasars, and \(\hbox {Ly}\alpha \) blobs. Because of their early formation, large spatial sizes and high total star-formation rates, protoclusters have also likely played a crucial role during the epoch of reionization, which can be tested with future experiments that will map the neutral and ionized cosmic web. Lastly, we review a number of promising observational projects that are expected to make significant impact in this growing, exciting field.

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Notes

  1. Despite the working definition of a protocluster that we will adopt here, there are many alternative views of what constitutes a protocluster that are, at the least, just as valuable. For example, a very conservative approach would be to call a massive or overdense high redshift structure a cluster only when it meets a minimum set of conditions typical for clusters (e.g., detection of a thermal ICM in the X-rays or a well-defined cluster red sequence), and anything else a protocluster. Other definitions could be constructed based on the redshift at which half of the present-day mass of a cluster was assembled (\(R_{1/2}\)) or on the redshift where environmental effects from a dense gaseous medium or galaxy interactions begin to alter significantly the properties of the infalling and orbiting cluster galaxies (e.g., ram-pressure stripping, tidal stripping, dynamical friction, and quenching).

  2. We should note that several of these objects would be classified as protoclusters if we follow the definitions given in Sect. 1.3. These objects have mass estimates based on either X-ray luminosity or velocity dispersion that formally lie below \(10^{14} M_\odot \) (e.g., see Pierre et al. 2012; Gobat et al. 2013; Yuan et al. 2014).

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Acknowledgements

I owe great gratitude to Arjun Dey, Avishai Dekel, Alessandro Rettura, Alvaro Orsi, Benedetta Ciardi, Brian Lemaux, Eduardo Bañados, Elizabeth Cooke, John Silverman, Jun Toshikawa, Kyoung-Soo Lee, Nina Hatch, Nobunari Kashikawa, Olga Cucciati, Ricardo Demarco, Sebastiano Cantalupo, Yi-Kuan Chiang, the editors, and the anonymous referees for valuable feedback that greatly improved this manuscript. I thank Lindsey Bleem for making available the cluster data used in Figs. 5, 7 and 11, and Mike Boylan-Kolchin, Benedetta Ciardi, Jun Toshikawa, Casey Stark, Yi-Kuan Chiang, Alexandro Saro, Sebastiano Cantalupo and Yusei Koyama for granting permission to reproduce their figures.

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Correspondence to Roderik A. Overzier.

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This project received support from CNPq (308192/2013-3; 459040/2014-6; 400738/2014-7), FAPERJ (111.404/2014; 202.876/2015), and the Visiting Scholar Program of the Research Coordination Committee of the National Astronomical Observatory of Japan (NAOJ).

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Overzier, R.A. The realm of the galaxy protoclusters. Astron Astrophys Rev 24, 14 (2016). https://doi.org/10.1007/s00159-016-0100-3

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  • DOI: https://doi.org/10.1007/s00159-016-0100-3

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