Distant cluster hunting: A comparison between the optical and X-ray luminosity functions from an optical/X-ray joint survey

We present a comparison of X-ray and optical luminosities and luminosity fu nctions of cluster candidates from a joint optical/X-ray survey, the ROSAT Optical X-Ray Survey. Completely independent X-ray and optical catalogs of 23 ROSAT fields (4.8 deg(2)) were created by a matched-filter optical algor ithm and by a wavelet technique in the X-ray. We directly compare the resul ts of the optical and X-ray selection techniques. The matched-filter techni que detected 74% (26 out of 35) of the most reliable cluster candidates in the X-ray-selected sample; the remainder could be either constellations of X-ray point sources or z > 1 clusters. The matched-filter technique identif ied approximately 3 times the number of candidates (152 candidates) found i n the X-ray survey of nearly the same sky (57 candidates). While the estima ted optical and X-ray luminosities of clusters of galaxies are correlated, the intrinsic scatter in this relationship is very large. We can reproduce the number and distribution of optical clusters with a model defined by the X-ray luminosity function and by an L-X-Lambda (cl) relation if H-0 = 75 k m s(-1) Mpc(-1) and if the L-X-Lambda (cl) relation is steeper than the exp ected L-X proportional to Lambda (2)(cl). On statistical grounds, a bimodal distribution of X-ray luminous and X-ray faint clusters is unnecessary to explain our observations. Follow-up work is required to confirm whether the clusters without bright X-ray counterparts are simply X-ray faint for thei r optical luminosity because of their low mass or youth or are a distinct p opulation of clusters that do not, for some reason, have dense intracluster media. We suspect that these optical clusters are low-mass systems, with c orrespondingly low X-ray temperatures and luminosities, or that they are no t yet completely virialized systems.