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