Weak lensing by high redshift clusters of galaxies

In this dissertation I study the weak gravitational lensing of background galaxies caused by massive high-redshift clusters of galaxies. I have selected a sample of six {dollar}z>0.5{dollar} X-ray selected clusters from the Einstein Medium Sensitivity Survey and the ROSAT North Ecliptic Pole Survey. The X-ray selection criteria ensures that the clusters are in fact massive and not merely superpositions of otherwise unrelated groups of galaxies. At the time I started this dissertation, these six clusters were the complete sample meeting the X-ray selection criteria.; I have obtained ultra-deep R-band images of these clusters with LRIS at the Keck observatory and moderately deep B and I-band images with the Tek2048{dollar}sp2{dollar} CCD at the UH88{dollar}sp{lcub}primeprime{rcub}{dollar} telescope. I have used the IMCAT data reduction package to measure the magnitudes, colors, and shapes of the galaxies and stars in the images. Background galaxies were then selected by magnitude and color and used in the weak lensing analysis.; I create two-dimensional maps of the surface density for each cluster using the KS93 algorithm. These massmaps tend to exhibit the same features as seen in the cluster galaxy distribution (cluster galaxies chosen by color and magnitude) and provide qualitative evidence for substructure in the clusters. I also measure one-dimensional radial surface density profiles using aperture densitometry and compare these to profiles of isothermal spheres and "universal" CDM models. I find that while the less massive clusters are in general well fit by isothermal spheres indicating that they are close to virialization, the more massive clusters are not as centrally compact, possibly indicating that they are in the process of collapse.; Finally, I compare the mass estimates from the X-ray data with the dimensionless surface densities from the weak lensing analysis and measure a median redshift for the background galaxies with {dollar}23