The realm of the galaxy protoclusters

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 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. Last, we review a number of promising observational projects that are expected to make significant impact in this growing, exciting field.Comment: Invited article for The Astronomy and Astrophysics Review (in press

The implications of the surprising existence of a large, massive CO disk in a distant protocluster

“…We note that our sample at z > 0.4 is dominated by protoclusters mem-bers. Thus, there could be a bias in our compilation as high-z protocluster are, by definition, the early stages in the evolution of clusters (see Overzier 2016, for more details on protoclusters).…”

Discovery of a Protocluster Associated with a Lyα Blob Pair at z = 2.3

“…Since protoclusters lack many of the observational properties of massive virialized galaxy clusters of today, one of the best ways to find them is to identify galaxy over-densities at high redshift (Overzier 2016 observable populations of galaxies include radio galaxies (Venemans et al 2002(Venemans et al , 2007Hatch et al 2011b;Hayashi et al 2012;Wylezalek et al 2013;Cooke et al 2014), submillimeter galaxies (Daddi et al 2009;Capak et al 2011;Rigby et al 2014;Dannerbauer et al 2014), Hydrogen Alpha Emitters (HAEs) (Hatch et al 2011a;Hayashi et al 2012), or Lyman Break Galaxies (LBGs) and Lyman Alpha Emitters (LAEs) (e.g., Taniguchi et al 2005;Overzier et al 2006Overzier et al , 2008. LAEs, which are compact galaxies that have strong emission in the Lyman-α line, are relatively easy to observe over a wide range of redshifts at z ∼ 2-6 (e.g., Taniguchi et al 2005;Gronwall et al 2007;Nilsson et al 2009;Guaita et al 2010).…”

Luminous quasars do not live in the most overdense regions of galaxies at z ∼ 4

“…Recently, Kikuta et al (2017) investigated quasar fields at z ∼ 4.9 to suggest that high-z quasars are not associated with extreme galaxy overdense regions. From the above, it is clear that a consistent picture of the environment of high-z quasars still needs to be derived (see Overzier 2016, for a review).…”