Galaxy evolution in a large sample of X-ray clusters

It is long established that the evolution of populations of galaxies is dependent upon the environment in which they are located, from low mass galaxy groups to rich galaxy clusters. However, what is not so clear is which physical process(es) dominate this evolution. There are a number of different mechanisms which have been proposed and these can be broadly divided into two categories. There are those caused by interactions with the cluster environment itself (including ram-pressure stripping) and those caused by galaxy-galaxy interactions (including merging events). Disentangling these is a non-trivial task. In this thesis, we use a number of parameters to do so all based upon our uniform CFHT Megacam photometry for X-ray selected galaxy clusters drawn from the X-Ray Multi-Mirror (XMM) Large Scale Structure (LSS) survey and the Canadian Cluster Comparison Project (CCCP). These clusters possess well determined X-ray temperatures spanning the range 1 < kT(keV) < 12 and occupy a relatively narrow redshift interval (0.15 < z < 0.41) in order to minimise any redshift dependent photometric effects. We investigate the colour bimodality of cluster galaxy populations and compute blue fractions using the criteria of Butcher and Oemler (1984) and identify a trend of observing increasing blue fractions versus redshift in common with numerous previous studies. However, we also identify an environmental dependence of cluster blue fraction in that cool (low mass) clusters display higher blue fractions than hotter (higher mass) clusters. Introducing the local galaxy density parameter, Sigma5, we find that there is a greater variation in blue fraction as a function of Sigma 5 in the low mass groups compared to the high mass clusters, but all of our samples (cool, mid and hot temperatures) show a decrease in blue fraction with an increase in local galaxy density, consistent with galaxy-galaxy interactions. We also show that the global cluster environment is playing a role since we observe that, at similar local galaxy densities, there is a greater decrease in the blue fraction as the cluster temperature increases. A further, important consideration is that of the timescales required for environmental effects to become effective. Through simple modelling, we find that our mid and hot samples have had sufficient halo mass for sufficient lengths of time for environmental mechanisms to act. We also observe that the value of fB does not depend strongly on the current state of the X-ray gas. Our studies of the red-sequence luminosity functions and the related dwarf-to-giant ratios (DGR) add further support to an emerging picture of galaxy-cluster and galaxy-galaxy interactions where we find that the dwarf population is produced via ram-pressure stripping and passive reddening before being converted into giants via the effects of merging. As one would expect, galaxy interactions (whether with other galaxies or the cluster environment) should have some impact upon the morphology. Using the GIM2D modelling package to determine morphological parameters, we observe an increase in the fraction of bulge-dominated galaxies with increasing local galaxy density but that the morphological mix responds less strongly to variations in global environment than does the colour mix. We also find that our bulge-to-total distributions show, in the cool sample, passively reddening disk galaxies prior to their disruption via merging with our distributions of disk scale lengths suggesting that the destruction of disks in the mid and hot samples must be a rapidly occurring process