AST 95-00515 PI: J. Patrick Henry The study of rich clusters of galaxies provides much information of cosmological importance. They were the first places where dark matter was found. Their brightest member galaxies have been used as standard candles from the inception of modern investigations into the age and fate of the Universe. Despite their extreme importance, there has never been a catalog of clusters at high redshifts that could be used for objective statistical studies because most clusters are found from visual inspection of optical plates. Clusters of galaxies are also very luminous X-ray emitters, hence they are found in X-ray surveys of the sky. Further, finding clusters by their X-ray emission properties greatly reduces biases inherent in selecting them in the visible band while simultaneously providing a catalog with quantifiable characteristics suitable for statistical analysis. The most sensitive X-ray survey of an appreciable region of the sky is that performed by the German satellite ROSAT around the North Ecliptic Pole (NEP). Work already performed under an existing NSF grant shows that the number of clusters at high redshifts in the X-ray data is as expected, and will in fact provide a sample of about the same size as that accumulated after decades of effort using optical techniques. This investigation will optically follow-up studies of the ROSAT NEP X-ray cluster sample with particular emphasis on the initial identifications of the clusters and later investigations of evolution of X-ray and optical properties, large scale structure, and the infrared magnitude-redshift diagram. The depth and completeness properties of the ROSAT sample will enable significant improvements to be made in these areas compared to previous studies. ***
