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Determination of the correlation length for a cluster of galaxies reveals that the clusters do not exhibit considerable elongation along the line of sight as predicted by theoretical calculations. The correlation length is measured as 13 to 15 per Hubble constant per megaparsecs. X-ray emission from these galaxies is used for the selection of galaxy clusters for the study. Optical research reveals that theoretical calculations of the large scale structure in the universe contradict the optically measured correlation length of the clusters of the galaxies.

The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all publicly available data in the XMM- Newton Science Archive. Our recent first data release (XCS-DR1) contains 503 optically confirmed groups and clusters, among which 256 new to the literature and 357 whose X-ray emission was detected for the first time. We discuss their properties and provide an update on the work being done. As examples of the applications of XCS-DR1, we mention the 17 fossil groups/clusters identified with the help of the Sloan Digital Sky Survey. Their brightest galaxies have stellar populations and star-formation histories which are similar to normal brightest cluster galaxies, but their stellar masses are significantly larger and correspond to a much bigger fraction of the total group/cluster optical luminosity. We also highlight the 15 clusters expected tobe also detected by the Planck satellite, and characterize the expected overlap between the final XCS and Planck cluster catalogues. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The redshift evolution of the galaxy cluster temperature function is a powerful probe of cosmology. However, its determination requires the measurement of redshifts for all clusters in a catalogue, which is likely to prove challenging for large catalogues expected from XMM-Newton, which may contain of the order of 2000 clusters with measurable temperatures, distributed around the sky. In this paper we study the apparent cluster temperature, which can be obtained without cluster redshifts. We show that the apparent temperature function itself is of limited use in constraining cosmology, and so concentrate our focus on studying how apparent temperatures can be combined with other X-ray information to constrain the cluster redshift. We also briefly study the circumstances under which the non-thermal spectral features can provide redshift information. [ABSTRACT FROM AUTHOR]

Presents a study on the galaxy clusters selected based on their x-ray emissions. Advantages of x-ray over optical observations; Method used in expressing egree clustering; Lack of evidence for clusters being significantly elongated along the line of sight contrary to previous suggestions.