Ancient Light From Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-Clusters

Abstract

A growing number of galaxy clusters at z =1-2 is being discovered as part of deep optical, IR, X-ray, and Sunyaev-Zel'dovich effect surveys. For a complete picture of cluster formation, however, it is important that we also start probing the much earlier epoch, between redshifts of about 2 and 7, during which these clusters and their galaxies first began to form. Because the study of these so-called "proto-clusters" is currently quite limited by small number statistics, widely varying selection techniques, and many assumptions, we have performed a large systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in about 3000 clusters from the earliest times to z = 0. We define an effective radius R e for proto-clusters and characterize their growth in size and mass with cosmic time. We show that the progenitor regions of galaxy clusters (ranging in mass from ∼ 10 14 to a few times 10 15 M ⊙ ) can already be identified in galaxy surveys at very early times (at least up to z ∼ 5), provided that the galaxy overdensities are measured on a sufficiently large-scale (R e ∼5-10 Mpc comoving) and with sufficient statistics. We present the overdensities in matter, dark matter halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the wide range of structures found in real surveys. We also derive the probability that a structure having a galaxy overdensity δ gal , defined by a set of observational selection criteria, is indeed a proto-cluster, and show how their z = 0 masses can already be estimated long before virialization. Galaxy overdensity profiles as a function of radius are presented. We further show how the projected surface overdensities of proto-clusters decrease as the uncertainties in redshift measurements increase. We provide a table of proto-cluster candidates selected from the literature, and discuss their properties in the light of our simulations predictions. This paper provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e.g., the upcoming HETDEX and Hyper Suprime-Cam surveys.

The environments of Ly α blobs – I. Wide-field Ly α imaging of TN J1338−1942, a powerful radio galaxy at z ≃ 4.1 associated with a giant Ly α nebula★

“…Since the TNJ1338 field has higher G than the whole simulation (and the SXDS field), the number density of TNJ1338-like field is thought to be close to that of such high-G fields. The number density is thus comparable to that of Coma-type protoclusters identified in the same simulation (58 protoclusters; Chiang et al 2013). The number (21 peaks) corresponds to number density of 6.4 × 10 −8 Mpc −3 in comoving units.…”

“…a protocluster (Venemans et al 2002(Venemans et al , 2007Miley et al 2004;Intema et al 2006;Overzier et al 2008). Theoretical simulations also suggests that the overdensity associated with TNJ1338 is likely to be an ancestor of a present-day cluster (Chiang, Overzier & Gebhardt 2013). Multiwavelength observations have shown that this source has radio lobes with an extent of ∼70 kpc.…”

“…This means that, if we assume the galaxy bias for LAEs at log L Lyα ∼ 43 to be b ∼ 2.4-4 (e.g. Ouchi et al 2005Ouchi et al , 2010Kovač et al 2007;Chiang et al 2013), and the faint LAEs are correctly tracing the matter density of this field, the bias for brighter LAEs must be b ∼ 20-40.…”

The impact of protocluster environments atz= 1.6

“…Beyond z ∼ 2.5 there are few massive groups within protoclusters (Chiang, Overzier & Gebhardt 2013), so we expect the environmental effects we observe at z = 1.6 to decrease at higher redshifts. The passive fraction of protoclusters at z > 2 is difficult to measure, but there is some evidence that suggests that the suppression of star formation within protoclusters decreases at z > 2 (Cooke et al 2014;Husband et al 2016;Wang et al 2016).…”

The structure and evolution of a forming galaxy cluster atz= 1.62

“…Recently progress towards this has been made by Chiang, Overzier & Gebhardt (2013) who demonstrate there is a strong correlation between the descendant z = 0 cluster mass and the mass overdensity of a protocluster over large volumes (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) diameter). The uncertainty of the descendant z = 0 cluster mass for each protocluster is very large, due to both intrinsic scatter in the relation and the inherent problems in converting the observed projected galaxy overdensity into the true 3D mass overdensity (Shattow et al 2013), but such methods can be successful in estimating the statistically likely mass of a large sample of protoclusters.…”