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M. E. Donahue (STScI), ROX Collaboration
We report on results from our complete optical and X-ray search for moderately distant clusters of galaxies (z=0.2-1.0) in deep ROSAT PSPC exposures. We use the Postman matched filter technique for identifying optical candidates from galaxy catalogs complete to I < 23.5 mag and the Rosati wavelet technique to identify X-ray luminous clusters of galaxies. We observed and analyzed 23 ROSAT fields. We have found 125 optical cluster candidates and 32 X-ray cluster candidates. By cross- identification we have been able to make a statistical study of the effects of the selection technique. We estimate the intrinsic dispersion of the relationship between the X-ray luminosity and the matched-filter estimate of the optical luminosity, \Lambdacl. We have discovered a candidate intergalactic filament of hot gas in one field and evidence for extended emission in other fields, using the angular cross-correlation measure between galaxies and the X-ray background not resolved into distinct sources by ROSAT. Such a technique will be a powerful analysis tool for XMM and Chandra fields. We discuss the future of X-ray background studies and the search for the warm baryon component of the universe by means of X-ray and optical techniques.