X-ray clusters of galaxies and the luminosity-richness relation.

Properties of the nearby clusters of galaxies (distance class 4 or less) identified with X-ray sources are investigated on the basis of Uhuru observations combined with Ariel 5 and OSO 7 results. General properties of the X-ray clusters, such as richness and morphological type, are discussed. X-ray spectra are presented for those clusters for which sufficient Uhuru observations are available. A fundamental relation between the clusters and their richness as defined optically by Abell is observed, such that the class of richness 2 clusters contains relatively more highly luminous X-ray sources as compared with richness 0 and 1 clusters. The observations are shown not to support the previously suggested correlations between X-ray luminosity and Bautz-Morgan or Rood-Shastry morphological type. It is proposed that the richness-luminosity correlation results from the greater number of galaxies available to produce and contain hot intergalactic gas.