EXOSAT observations of clusters of galaxies - II. X-ray to optical correlations

The results obtained for clusters of galaxies using X-ray data from the European X-ray Observatory Satellite, EXOSAT, are compared with the optical, infrared and radio properties of the clusters taken from the literature. A number of strong correlations are found. The ratios of the intracluster medium (ICM) temperature and the cluster velocity dispersion, β, have a mean of less than one, in agreement with values obtained from the surface brightness distribution of the X-ray emission, thus resolving the ‘Beta problem’. A particularly strong correlation exists between X-ray luminosity and spiral fraction indicating a direct link between the ICM and the structure of galaxies. The luminosity of the brightest cluster member and the X-ray properties of the cluster are found to be related, implying that its use as a standard candle should be corrected for the X-ray properties. The correlations between galaxy number density and X-ray properties imply mass-to-light ratios of $$100-250 \enspace h M_\odot/L_\odot$$ in the cores of clusters and a ratio of gas mass to galactic mass of between one and two for all clusters. The implications of these results are reviewed.