Distant Cluster Hunting II: A Comparison of X-ray and Optical Cluster Detection Techniques and Catalogs from the ROX Survey

We present and analyze the optical and X-ray catalogs of moderateredshift cluster candidates from the ROSAT Optical X-ray Survey, or ROXS. The survey covers the sky area contained in the fields of view of 23 deep archival ROSAT PSPC pointings, 4.8 square degrees. The crosscorrelated cluster catalogs were constructed by comparing two independent catalogs extracted from the optical and X-ray bandpasses, using a matched-filter technique for the optical data and a wavelet technique for the X-ray data. We cross-identified cluster candidates in each catalog. As reported in Paper I, the matched-filter technique found optical counterparts for at least 60% (26 out of 43) of the X-ray cluster candidates; the estimated redshifts from the matched filter algorithm agree with at least 7 of 11 spectroscopic confirmations (∆z . 0.10). The matched filter technique, with an imaging sensitivity of mI ∼ 23, identified approximately 3 times the number of candidates (155 candidates, 142 with a detection confidence > 3σ) found in the X-ray survey of nearly the same area. There are 57 X-ray candidates, 43 of which are unobscured by scattered light or bright stars in the optical images. Twenty-six of these have fairly secure optical counterparts. We find that the matched filter algorithm, when applied to images with galaxy flux sensitivies of Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, [email protected] Columbia University, Columbia Astrophysics Lab, Mail Code 5247 Pupin Hall, 550 W. 120th St., New York, NY 10027 European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching, D-85748 University of Colorado, CASA, CB 389, Boulder, CO 80309 Visiting Astronomer, Kitt Peak National Observatory