What are protoclusters? – Defining high-redshift galaxy clusters and protoclusters

Abstract

We explore the structures of protoclusters and their relationship with high redshift clusters using the Millennium Simulation combined with a semi-analytic model. We find that protoclusters are very extended, with 90 per cent of their mass spread across ∼ 35 h −1 Mpc comoving at z = 2 (∼ 30 arcmin). The 'main halo', which can manifest as a high redshift cluster or group, is only a minor feature of the protocluster, containing less than 20 per cent of all protocluster galaxies at z = 2. Furthermore, many protoclusters do not contain a main halo that is massive enough to be identified as a high redshift cluster. Protoclusters exist in a range of evolutionary states at high redshift, independent of the mass they will evolve to at z = 0. We show that the evolutionary state of a protocluster can be approximated by the mass ratio of the first and second most massive haloes within the protocluster, and the z = 0 mass of a protocluster can be estimated to within 0.2 dex accuracy if both the mass of the main halo and the evolutionary state is known. We also investigate the biases introduced by only observing star-forming protocluster members within small fields. The star formation rate required for line-emitting galaxies to be detected is typically high, which leads to the artificial loss of low mass galaxies from the protocluster sample. This effect is stronger for observations of the centre of the protocluster, where the quenched galaxy fraction is higher. This loss of low mass galaxies, relative to the field, distorts the size of the galaxy overdensity, which in turn can contribute to errors in predicting the z = 0 evolved mass.

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

“…The fraction of galaxies in the most massive group is an upper limit as there are likely to be additional protocluster galaxies beyond the observed window. Protoclusters extend over several Mpc so the observations presented here are not likely to encompass the entire protocluster structure (Muldrew, Hatch & Cooke 2015).…”

“…These are agglomerations of galaxies and groups that will merge to form a cluster by the present day. These bound structures are very extended, stretching up to 50 co-moving Mpc in diameter (Muldrew, Hatch & Cooke 2015). Following the definition of a cluster as a virialized structure, we therefore refer to the most massive halo of the protocluster as the high-redshift cluster.…”

“…Fassbender et al 2014), and contain only a small fraction of the galaxies that will eventually form the presentday cluster. Cosmological simulations reveal that the majority of ⋆ E-mail: nina.hatch@nottingham.ac.uk galaxies in clusters at z = 0 did not reside in the main halo at z > 1, but rather in the extended protocluster (Muldrew, Hatch & Cooke 2015). It is therefore essential to study the protocluster, and not simply the main halo, to trace the evolution of cluster galaxies to the highest redshifts.…”

LARGE-SCALE STRUCTURE AROUND A z = 2.1 CLUSTER

“…Within a 10′ (corresponds to a proper distance of ∼5 Mpc) radius from the ZFIRE cluster center, the redshift distribution of SFGs from the zCOSMOS survey peaks at similar redshift (median = , the error represents the bootstrapped uncertainties) as the ZFIRE cluster members. We perform the FoF analysis (Huchra & Geller 1982) to determine which galaxies from the zCOSMOS, SPIRE/SCUBA 2 surveys and MOSDEF/ VUDS can be linked to the ZFIRE cluster members (from redshift survey (1); blue dots in Figure 2) with a linking length of 2 Mpc, a scale that accommodates the large spatial extent of unvirialized structure at > z 2 (e.g., Chiang et al 2013;Muldrew et al 2015). The structure identified from the FoF algorithm (black circles in Figure 2) spans a comoving volume of ∼25,000 Mpc 3 , and its spatial extent is consistent with the z = 2.07 structure identified based on photometric redshifts (Chiang et al 2014).…”

“…lack of an excess of massive galaxies in the protocluster environment may be a bias introduced by primarily selecting star-forming populations (e.g., Muldrew et al 2015). Despite the fact that protocluster and field SFGs have similar mean SFR, their SFR cumulative distributions show a slight discrepancy in which the cluster members are composed with slightly more high-SFR galaxies but fewer low-SFR galaxies (the p-value from the K-S test is 0.04).…”