Results of ASCA X-ray study of central regions of medium-richness clusters of galaxies are summarized, emphasizing differences between cD and non-cD clusters. The intra-cluster medium (ICM) is likely to consist of two (hot and cool) phases within 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 little metallicity gradient right to the center. The gravitational potential exhibits a hierarchical nesting around cD galaxies, while a total mass-density profile with a central cusp is indicated for a non-cD cluster Abell 1060. The ironmass-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 abundance ratio in the ICM increases with the cluster richness, but remains close to the solar ratio around cD galaxies. These overall results are interpreted without appealing to the popular cooling-flow hypothesis. Instead, an emphasis is put on the halo-in-halo structure formed around cD galaxies.

Using the X-ray data from ASCA, spectral and spatial properties of the intracluster medium (ICM) of the cD cluster Abell 1795 are studied, up to a radial distance of～12′(～1.3h−1 50kpc). The ICM temperature and abundance are spatially rather constant, although the cool emission component is reconfirmed in the central region. The azimuthally-averaged radial X-ray surface brightness profiles are very similar between soft (0.7-3 keV) and hard (3-10 keV) energy bands, and neither can be fitted with a single-model due to a strong data excess within～5′ of the cluster center. In contrast, double-models can successfully reproduce the overall brightness profiles both in the soft and hard energy bands, as well as that derived with the ROSAT PSPC. Properties of the central excess brightness are very similar over the 0.2-10 keV energy range spanned by ROSAT and ASCA. Thus, the excess X-ray emission from the core region of this cluster is confirmed for the first time in hard X-rays above 3 keV. This indicates that the shape of the gravitational potential becomes deeper than the Kingtype one towards the cluster center. Radial profiles of the total gravitating matter, calculated using the double-model, reveal an excess mass of～3 × 1013 M⊙ within～150 h−1 50 kpc of the cluster center. This suggests a hierarchy in the gravitational potential corresponding to the cD galaxy and the entire cluster.