This thesis is comprised of X-ray and optical studies of 27 X-ray selected galaxy clusters from the XMM-LSS survey. These systems are mostly groups and poor clusters, with temperatures 0.6-4.8 keV, spanning the redshift range 0.05 to 1.05, hence these are some of the highest redshift X-ray selected clusters to have been studied. In the X-ray study, the evolution in the X-ray surface brightness profiles of the hot intracluster plasma is studied. Comparing the profiles with a standard -model it is found that 54% of the sample possess cuspy (cool) cores. Trends with both temperature and redshift in the outer slope beta of the X-ray surface brightness and in the incidence of cuspy cores are investigated. Fits indicate that the incidence of cuspy cores does not decline at high redshifts, as has been reported in rich clusters. Rather such cores become more prominent with increasing redshift. It is also found that has a positive correlation with temperature. In the optical study, CFHTLS optical photometry has been used to study the galaxy luminosity functions of 14 members of the sample. Individual luminosity functions (LFs) as well as redshift-stacked and temperature-stacked LFs in three filters, g', r' and z', down to M = -14:5 are derived. All LFs were fitted by Schechter functions which well-constrained the faint-end slope, . Derived values of ranged from -:03 to as steep as -2:1. No evidence is found for upturns at faint magnitudes. Evolution in was apparent in all bands: it becomes shallower with increasing redshift. It is found that at z~0:3, alpha is steeper (-1.67) in the green (g') band than it is (-1.30) in the red (z') band. This colour trend disappears at low redshift, which is attributed to reddening of faint blue galaxies from z ~0.3 to z~0. Also, the total optical luminosity is calculated and is found to correlate strongly with X-ray luminosity and temperature, which is consistent with expectations for self-similar clusters with constant mass-to-light ratio
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