Gravitational lensing by X-ray luminous galaxy clusters
Since the discovery that the large-scale dynamics of galaxy clusters are dominated by dark matter, cosmologists have aspired to measure the spatial distribution of dark matter and identify its nature. Gravitational lensing, especially employing the Hubble Space Telescope (HST) has emerged as the tool-of-choice for mapping dark matter. Standing on the shoulders of the pioneering 1990's, this thesis is the first homogeneous lensing study of clusters with HST. We measure the mass and structure of an objectively-selected sample of X-ray luminous clusters at a single epoch (z ~ 0.2). We present observations often clusters (L(_x)≥8x10(^44)[0.1=2.4 keV] ergs(^-1)) and use the numerous gravitationally-lensed features in these data to constrain a detailed model of the central regions (r ~ 500 kpc) of each cluster. Our models provide an unprecedented view of cluster morphology, revealing that 60% of the sample contain significant substructure. Chandra X-ray observations confirm this is a signature of dynamical immaturity, and show that the mean temperature of the intra-cluster medium of the morphologically complex clusters is ~ 25% higher than then regular siblings. This offset results in a critical, and previously unexplored, systematic uncertainty in the use of clusters to normalise the mass power spectrum. We also use the detailed morphology of the clusters to constrain the nature of dark matter. We then exploit the clusters as gravitational telescopes, using ground-based near-infrared imaging to construct a sample of 60 gravitationally magnified Extremely Red Objects (EROs), a population that is believed to harbour important clues on the formation epoch and mechanism of massive galaxies. This unique sample overcomes the faintness of EROs (R ≥ 23, K ≥ 18) to uncover a wealth of morphological, photometric and spectroscopic evidence of diversity in both passively evolving and dusty active EROs. Coupled with our deep number counts (to K ~ 22), these observations provide important new constraints on competing theoretical models of galaxy formation.