Galaxy evolution in a large sample of X-ray clusters
Date
2013-12-17
Authors
Urquhart, Sheona Anne
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Abstract
The evolution of galaxy populations is dependent upon the environment in which they are located, from low mass galaxy groups to rich galaxy clusters. However, what remains unclear is which physical process(es) dominate this evolution. We investigate this using uniform CFHT Megacam photometry for X-ray selected galaxy clusters from the X-Ray Multi-Mirror (XMM) Large Scale Structure (LSS) survey and the Canadian Cluster Comparison Project (CCCP). These clusters possess X-ray temperatures of 1<kT(keV)<12 and occupy a redshift interval 0.15<z<0.41 to minimise any redshift dependent photometric effects. We investigate the colour bimodality of cluster galaxy populations and compute blue fractions, identifying a trend of increasing blue fraction versus redshift. We also identify an environmental dependence of cluster blue fraction with cool clusters displaying higher values than hotter clusters. Using the local galaxy density parameter, ∑5, we find a greater variation in blue fraction as a function of ∑5 in low mass groups compared to high mass clusters, but all samples show a decrease in blue fraction with increasing local galaxy density, consistent with galaxy-galaxy interactions. Global cluster environment is also playing a role, at similar local galaxy densities, there is a greater decrease in blue fraction as cluster temperature increases. Through simple modelling, we find that our mid and hot samples have had large enough halo masses for sufficient lengths of time for environmental mechanisms to act and observe that the value of fB does not depend strongly on the current state of the X-ray gas. Our dwarf-to-giant ratios 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 conversion into giants via the effects of merging. 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, however, the morphological mix responds less strongly to variations in global environment than does the colour mix.
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Galaxy evolution