X-ray study of 79 distant clusters of galaxies: Discovery of two classes of cluster size

We have performed a uniform analysis of 79 clusters of galaxies with the ROSAT/High Resolution Imager (HRI) and ASCA to study the X-ray structure and evolution of clusters in the redshift range 0.1 < z < 1. We determined the average X-ray temperatures and the bolometric luminosities with ASCA and the spatial distributions of the X-ray brightness with ROSAT/HRI by utilizing the isothermal beta model. We do not find any significant redshift dependence in the X-ray parameters, including the temperature, beta-model parameters, and the central electron density. Among the parameters, the core radius shows the largest cluster-to-cluster dispersions. We discovered that the histogram of the core radius shows two peaks at 60 and 220 kpc. If we divide the cluster samples into two subgroups corresponding to the two peaks in the core radius distribution, they show differences in the X-ray and optical morphologies and in the X-ray luminosity-temperature relation. From these observational results, we suggest that the clusters are divided into at least two subgroups according to the core radius.