A New Method For Galaxy Cluster Detection. I. The Algorithm

Abstract

Numerous methods for finding clusters at moderate to high redshifts have been proposed in recent years, at wavelengths ranging from radio to X-rays. In this paper we describe a new method for detecting clusters in two-band optical/near-IR imaging data. The method relies upon the observation that all rich clusters, at all redshifts observed so far, appear to have a red sequence of early-type galaxies. The emerging picture is that all rich clusters contain a core population of passively evolving elliptical galaxies which are coeval and formed at high redshifts. The proposed search method exploits this strong empirical fact by using the red sequence as a direct indicator of overdensity. The fundamental advantage of this approach is that with appropriate filters, cluster elliptical galaxies at a given redshift are redder than all normal galaxies at lower redshifts. A simple color cut thus virtually eliminates all foreground contamination, even at significant redshifts. In this paper, one of a series of two, we describe the underlying assumptions and basic techniques of the method in detail, and contrast the method with those used by other authors. We provide a brief demonstration of the effectiveness of the technique using a real photometric sample with redshift data, and from this conclude that the method offers a powerful yet simple way of identify galaxy clusters. We find that the method can reliably detect structures to masses as small as groups with velocity dispersions of only ∼ 300 km s −1 , with redshifts for all detected structures estimated to an accuray of ∼10%.

Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000

“…Their ubiquitous colour-magnitude sequence, first noted locally by Baum (Baum 1959), is now well documented in many clusters, ranging from Coma to clusters at redshifts up to z = 1 (Stanford, Eisenhardt, & Dickinson 1998, Ellis et al 1997, Gladders & Yee 2000. The colours of elliptical galaxies become bluer as they become less luminous.…”

Correlating the CMB with luminous red galaxies: The integrated Sachs-Wolfe effect

“…The usefulness of LRGs as a cosmological probe has been appreciated by a number of authors [30,42]. These are typically the most luminous galaxies in the universe, and therefore probe cosmologically interesting volumes.…”

SPIDERS: the spectroscopic follow-up of X-ray-selected clusters of galaxies in SDSS-IV

“…Whilst multifilter optical imaging proves efficient at detecting and characterizing galaxy clusters -notably through their ubiquitous red sequence (e.g. Gladders & Yee 2000;Rykoff et al 2014) -ultimate confirmation of a galaxy cluster is achieved by optical spectroscopy. Spectroscopic observations of cluster members can be used to disentangle projection effects and substructures from real concentrations, and they also provide the precise redshift of the halo, and therefore lead to precise luminosities and masses once they are combined with X-ray measurements.…”