Red galaxy overdensities and the varied cluster environments of powerful radio sources with z   1.6

Abstract

The environments of a complete subsample of six of the most powerful radio-loud active galactic nuclei (AGN) at redshifts z ∼ 1.6 are investigated, using deep RJK imaging to depths of R ∼ 26, J ∼ 22.4 and K ∼ 20.6. An excess of galaxy counts in the K band is seen across these fields; these surplus galaxy counts are predominantly associated with red galaxies (R − K 4) of magnitudes 17.5 K 20.5 found within radial distances of ∼1 Mpc of the AGN host. These are exactly (though not uniquely) the magnitudes, colours and locations that would be expected of old passive elliptical galaxies in cluster environments at the redshifts of these AGN. Using both an Abell-style classification scheme and investigations of the angular and spatial cross-correlation functions of the galaxies, the average environment of the fields around these AGN is found to be consistent with Abell cluster richness classes 0 and 1. The amplitude of the angular cross-correlation function around the AGN is shown to be a strong function of galaxy colour and is highest when only those galaxies with the colours expected of old elliptical galaxies at these redshifts are considered.The images cover a relatively wide field, 5 × 5 arcmin 2 , allowing the distribution of the surplus galaxy counts to be investigated. The galaxy overdensities are found on two scales around the AGN: (i) pronounced central concentrations on radial scales within ∼150 kpc; where present, these are composed almost entirely of red (R − K 4) galaxies, suggesting that the morphology-density relation is imprinted into the centres of clusters at a high redshift;(ii) weaker large-scale excesses extending out to between 1 and 1.5-Mpc radius. The presence or absence of galaxy excesses on these two scales, however, differs greatly between the six different fields: the fields of two AGN do show red galaxy excesses on both scales, another two fields show only a large-scale red galaxy overdensity with no pronounced central concentration, and one field shows only a sharp central peak of red galaxies with no large-scale overdensity. The final field shows little evidence for an excess on any scale; this field is associated with an unresolved radio source, perhaps indicating that only extended radio sources probe cluster environments.Clearly, there is a large range in both the richness and the degree of concentration of any clustering environments around these distant AGN. The implications of this for both cluster formation and the nature of high-redshift AGN are discussed.

A Radio-to-mm Census of Star-forming Galaxies in Protocluster 4C23.56 at Z = 2.5: Gas Mass and Its Fraction Revealed with ALMA

“…Historically, radio galaxies have been targeted in a search for (proto)clusters since their hosts are the most massive galaxies (Seymour et al 2007) and are expected to be embedded in the most massive halos (e.g., Rocca-Volmerange et al 2004;Orsi et al 2016). Indeed, the method has successfully yielded promising results to find (proto)clusters (e.g., Le Fevre et al 1996;Kurk et al 2000;Best et al 2003;De Breuck et al 2004;Overzier et al 2006;Venemans et al 2007;Hatch et al 2011) and protocluster 4C23.56 is also one of them.…”

An excess of dusty starbursts related to the Spiderweb galaxy

“…The questions of when and how present-day galaxy clusters formed at high redshift have driven extensive searches for protoclusters of galaxies in the distant Universe in the past two decades (e.g., Le Fevre et al 1996;Steidel et al 1998;Pentericci et al 2000;Kurk et al 2000Kurk et al , 2004aBest et al 2003;Matsuda et al 2005;Daddi et al 2009a;Galametz et al 2010;Galametz et al 2012;Hatch et al 2011a,b;Mayo et al 2012;Walter et al 2012;Wylezalek et al 2013). Powerful high-redshift radio galaxies (HzRGs; see the review by Miley & De Breuck 2008) LABOCA and VLA images (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A55 are considered to be the most promising signposts of the most massive clusters in formation.…”

GMASS ultradeep spectroscopy of galaxies at z ~ 2

“…For the overdensities of galaxies known at high redshift, this is often problematic because their cluster members are selected on the basis of either their star formation activity (blue members, e.g., Kurk et al 2000;Pentericci et al 2000;Venemans et al 2002;Steidel et al 2005), or lack of it (red members, e.g., Best et al 2003;Kodama et al 2007;McCarthy et al 2007). The most well studied high-redshift systems confirmed to be clusters on the basis of their X-ray emission, are found between z = 1.0 and z = 1.4, redshifts for which spectroscopic data of both the red and blue populations can be practically acquired (e.g., at z = 1.26, Demarco et al 2007; Mei et al 2006).…”