X-ray probing of the central regions of clusters of galaxies

The results on ASCA X-ray study of the central regions of medium-richness c lusters of galaxies are summarized, while emphasizing the differences betwe en cD and non-cD clusters. The intra-cluster medium (ICM) is likely to cons ist of two (hot and cool) phases within similar to 100 kpc of a cD galaxy, where the ICM metallicity is also enhanced. In contrast, the ICM in non-cD clusters appears to be isothermal with a small metallicity gradient right o f the center. The gravitational potential exhibits a hierarchical nesting a round cD galaxies, while a total mass-density profile with a central cusp i s indicated for a non-cD cluster, Abell 1060. The iron-mass-to-light ratio of the ICM decreases toward the center in both types of clusters, although it is radially constant in peripheral regions. The silicon-to-iron abundanc e ratio in the ICM increases with the cluster richness, but remains close t o the solar ratio around cD galaxies. These overall results are interpreted without appealing to the popular cooling-flow hypothesis. Instead, an emph asis is put on the halo-in-halo structure formed around cD galaxies.