| Summary: | Clusters of galaxies are among the most luminous X-ray sources in the sky. The X-ray emission originates from hot, diffuse intracluster gas which dominates the baryonic matter in the cluster. The distribution of the gas density follows that of the underlying (dark) mass potential. Consequently the cluster X-ray emission is concentrated towards the central core region of the cluster. By contrast, the galaxy distribution in clusters is more extended and, when studied in projection (<I>e.g</I>. on photographic plates), provides a less pronounced (optical) luminosity enhancement in the cluster core. Traditionally cluster samples have been developed by searching for projected galaxy enhancements in photographic or CCD surveys. However, for the reason outlined above, optical selection is an inferior method to X-ray selection. The work described in this thesis concerns the development of the SGP RASS Cluster Sample (SRCS) and its application to a quantitative study of large-scale structure in the Universe. The SRCS has been constructed from data acquired during the ROSAT All Sky Survey (RASS), where ROSAT is the German/UK/US X-ray satellite that was launched in 1990. The RASS, completed in 1992, constitutes the first all sky survey made with an imaging X-ray telescope. Approximately 50,000 X-ray emitting astronomical objects were detected during the RASS, of which roughly 10% are clusters of galaxies. Until the advent of the RASS, it was not possible to make large X-ray selected samples of clusters because the existing X-ray data either suffered from poor spatial resolution or did not provide homogeneous coverage over a large enough area of the sky. The prospect, therefore, of samples of 1000 ROSAT detected X-ray clusters is very exciting. The astrophysical and cosmological problems that can be addressed with such samples are manifold.
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