Wavelength Doesn't Matter: Optical vs. X-ray Luminosities of Galaxy Clusters

We examine the relationship between the total X-ray and optical luminosities of groups and clusters of galaxies taken from various samples in the literature. The clusters and groups were drawn from four different catalogs: (1) the Abell/ACO catalog, (2) the Edinburgh-Durham Cluster Catalog, (3) the RASS Bright Cluster Sample, and (4) galaxy groups selected from the CfA redshift survey. These catalogs represent a significant cross-section of cluster selection tchniques as well as a wide range in mass scale and can be considered statistically independent. We have calculated new cluster X-ray and optical luminosities if not available in the literature. Based on 126 systems, our analysis shows that the total optical luminosity of a cluster is directly proportional to the total X-ray luminosity over a wide mass range and across all four cluster samples studied herein. We also show that total cluster optical luminosity is a good indicator of cluster virial mass, but richness is not. Our results suggest that (1) the selection method of galaxy clusters may not be crucial, providing wider latitude in assembling catalogs, (2) the luminosity per baryon does not vary systematically between systems ranging from groups to very rich clusters. We propose that future optical catalogs of clusters use the total optical luminosity of a cluster, instead of galaxy richness, as luminosity appears to be a better measure of cluster mass and can be directly related to X-ray catalogs of clusters. This will also facilitate the first direct comparison between the growing number of X-ray and optical catalogs.