X-ray Emission From Poor Clusters of Galaxies

In this paper we present x-ray observations of 25 poor clusters and compare their x-ray, optical, and radio properties with those of rich clusters. The x-ray morphology has been classified in a manner similar to that developed for rich clusters by Jones & Forman [ApJ, 276,38 (1984)] with 13 of the 18 detected clusters in our sample being D-type clusters, where the peak coincides with a dominant galaxy. For the five T-type clusters, the x-ray emission appears to trace the optical distribution of galaxies. In both classes a sizeable fraction of the x-ray emission is associated with individual galaxies. The two morphological classes have a similar range of optical properties, which suggests that they are of comparable mass and dynamical age. Poor clusters are consistent with being a scaled-down version of rich clusters in essentially all characteristics and the 13 clusters in our sample that are also common to the Morgan et al. [ApJ, 199, 545 (1975)] sample are found to be indistinguishable from the rest of the sample. The optical properties of seven clusters that were not detected at x-ray wavelengths are also indistinguishable from those of the x-ray detected clusters. This suggests that all poor clusters have a hot intracluster medium. For clusters that have radially symmetric x-ray emission and sufficient x-ray photon statistics, analyses of their surface- brightness profiles show them to have smaller core radii than rich clusters and, in one case, a central x-ray excess which may signify a cooling flow. Radio observations show that on average T-type clusters contain more radio sources than D-type clusters.