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.

We present a new investigation of the mass-temperature (Mtot-TX) relation of 22 nearby clusters based on the analysis of their ROSAT X-ray surface brightness profiles (SX(r)) and their ASCA emission weighted temperatures. Two methods of the cluster mass estimations are employed and their results are compared: (1) the conventional model for gas distribution along with the isothermal and hydrostatic equilibrium assumptions, and (2) the NFW profile for dark matter distribution whose characteristic density and length are determined by the observed SX(r). These two models yield essentially the same goodness of fits for SX(r) and the similar Mtot-TX relations, with the latter demonstrating a significant departure from the simple gravitational scaling of Mtot ∝ T3/2 X. It is also shown that the best-fit Mtot-TX relations could be reconciled with the theoretical expectation if the low-temperature clusters (TX<3.5 keV) are excluded from the list, which lends support to the scenario that the intracluster medium is preheated in the early phase of cluster formation. Together with the entropy-temperature distribution, the existence of a similarity break at TX=3−4 keV in the dynamical scaling relations for galaxy clusters has been confirmed.

We present a study of the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing the high quality Chandra and ROSAT X-ray spectroscopic data. In order to probe the stellar mass distribution we calculated the B-band mass-to-light ratio profile by comparing the observed line-strength indices and multi-color photometric data with different stellar synthesis model predictions. Based on the recent survey results we also have modeled the contribution from other group members to the stellar mass. We find that the gas is single-phase with a temperature of 0.7 keV within 1Re (1Re=9.0 kpc), which is typical for elliptical galaxies. Outside 1Re the gas temperature increases quickly outwards to>1 keV, indicating its group origin. With the high spatial resolution of Chandra we reveal that the X-ray surface brightness profile shows a central excess in the innermost region, and on both the total mass and dark matter profiles there is a flattened feature at about<1Re, which coincides with the gas temperature transition from the galactic level to the group level. We speculate that this may be a mark of the boundary between the galaxy and group halos, as has been seen in some other cluster/group-dominating galaxies. The total mass and dark matter distributions within 0.85Re are cuspy and can be approximated by power-law profiles with indices of 2, which are marginally consistent with the generalized NFW profiles with=2. The mass in outer regions can be well fitted by a single NFW profile, and the derived concentration parameter c (18.6±1.5) is larger than the 68% upper limit for a halo at z=0 with the given Mvir. We find that the NGC 1407 group has a baryondominated core, while the mass in the>1Re is dominated by dark matter. At the virial radius r200 = 572±118 kpc, the inferred mass and mass-to-light ratio are M200=2.20±0.42×1013 M⊙ and Mvir/LB=311±60 M⊙/LB,⊙, respectively, showing that the NGC 1407 group is an extremely dark system even comparable to many clusters of galaxies. Since the obtained total mass is lower than those given in the earlier galaxy kinematic works, we speculate that NGC 1400 is not a virialized member in the group’s gravitational potential well.

We present the first high spectral resolution X-ray observation of the giant elliptical galaxy NGC 4636, obtained with the Reflection Grating Spectrometer on-board the XMM-Newton Observatory. The resulting spectrum contains a wealth of emission lines from various charge states of oxygen, neon, magnesium, and iron. Examination of the cross-dispersion profiles of several of these lines provides clear, unambiguous evidence of resonance scattering by the highest oscillator strength lines, as well as a weak temperature gradient in the inner regions of the interstellar medium. We invoke a sophisticated new Monte Carlo technique which allows us to properly account for these effects in performing quantitative fits to the spectrum. Our spectral fits are not subject to many of the systematics that have plagued earlier investigations. The derived metal abundances are higher than have been inferred from prior, lower spectral resolution observations of this source (Awaki et al. 1994), but are still incompatible with conventional chemical enrichment models of elliptical galaxies. In addition, our data are incompatible with standard cooling flow models for this system-our derived upper limit to the mass deposition rate is below the predicted value by a factor of 3-5.

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.