SIMBAD references

For a detailed comparison of the appearance of cluster of galaxies in X-rays and in the optical, we have compiled a comprehensive database of X-ray and optical properties of a sample of clusters based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS). The X-ray galaxy clusters of this RASS-SDSS catalog cover a wide range of masses, from groups of 10^12.5M_☉ to massive clusters of 10¹⁵M_☉ in the redshift range 0.002-0.45. The RASS-SDSS sample comprises all the X-ray selected objects already observed by the Sloan Digital Sky Survey (114 clusters). For each system we have uniformly determined the X-ray (luminosity in the ROSAT band, bolometric luminosity, center coordinates) and optical properties (Schechter luminosity function parameters, luminosity, central galaxy density, core, total and half-light radii). For a subsample of 53 clusters we have also compiled the temperatures and the iron abundances from the literature. The total optical luminosity can be determined with a typical uncertainty of 20% independent of the choice of local or global background subtraction. We searched for parameters which provide the best correlation between the X-ray luminosity and the optical properties and found that the z band luminosity determined within a cluster aperture of 0.5Mpc/h₇₀ provides the best correlation, with a scatter of about 60-70%. The scatter decreases to less than 40% if the correlation is limited to the bright X-ray clusters. The resulting correlation of L_X and L_op in the z and i bands shows a logarithmic slope of 0.38, a value not consistent with the assumption of a constant M/L. Consistency is found, however, for an M/L increasing with luminosity as suggested by other observations. We also investigated the correlation between L_op and the X-ray temperature, obtaining the same result.