The XMM-LSS survey: optical assessment and properties of different X-ray selected cluster classes

TL;DR

This paper presents an optical assessment of 66 X-ray selected galaxy clusters from the XMM-LSS survey, analyzing their properties, classifications, and relations to the cosmic web, with a focus on low mass and distant clusters.

Contribution

It introduces a new classification method for X-ray clusters based on optical and X-ray data, and assesses their reality and properties in a large survey area.

Findings

Most clusters are low mass and poorly studied.

Correlations found between optical richness and X-ray luminosity.

Identification of distant clusters and fossil groups.

Abstract

XMM and Chandra opened a new area for the study of clusters of galaxies. Not only for cluster physics but also, for the detection of faint and distant clusters that were inaccessible with previous missions. This article presents 66 spectroscopically confirmed clusters (0.05<z<1.5) within an area of 6 deg2 enclosed in the XMM-LSS survey. Almost two thirds have been confirmed with dedicated spectroscopy only and 10% have been confirmed with dedicated spectroscopy supplemented by literature redshifts. Sub-samples, or classes, of extended-sources are defined in a two-dimensional X-ray parameter space allowing for various degrees of completeness and contamination. We describe the procedure developed to assess the reality of these cluster candidates using the CFHTLS photometric data and spectroscopic information from our own follow-up campaigns. Most of these objects are low mass clusters, hence constituting a still poorly studied population. In a second step, we quantify correlations between the optical properties such as richness or velocity dispersion and the cluster X-ray luminosities. We examine the relation of the clusters to the cosmic web. Finally, we review peculiar structures in the surveyed area like very distant clusters and fossil groups.