X-Ray Probing of the Central Regions of Clusters of Galaxies

Kazuo Makishima,Hajime Ezawa,Takayuki Tamura,Tuneyoshi Kamae,Yasushi Fukazawa,Takaya Ohashi,Hirohiko Honda,Yasushi Ikebe,Kyoko Matsushita,Ken’Ich Kikuchi,Kazuhiro Nakazawa,Tadayuki Takahashi,Haiguang Xu

doi:10.1093/pasj/53.3.401

Abstract

Abstract The results on ASCA X-ray study of the central regions of medium-richness clusters of galaxies are summarized, while emphasizing the differences between cD and non-cD clusters. The intra-cluster medium (ICM) is likely to consist of two (hot and cool) phases within $\sim 100$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 of 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 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 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.

Highlights

Central regions of galaxy clusters are of particular interest with respect to the formation and evolution of galaxies and clusters
We have challenged this issue through ASCA observations of nearby clusters, and arrived at a new picture of the gas and mass structure of the central regions of clusters
By combining several observational works on central regions of galaxy clusters achieved with ASCA, we have arrived at a novel view therein

Summary

Introduction

Central regions of galaxy clusters are of particular interest with respect to the formation and evolution of galaxies and clusters. At or near the center of some clusters, we often find a single dominant elliptical galaxy (cD galaxy). Schombert 1986; Johnstone et al 1991), these objects form by far the most luminous class of galaxies. Extensive theoretical and numerical studies generally suggest that they were formed relatively early in the course of cluster evolution (e.g., Merritt 1985; Dubinski 1998; Ghigna et al 2000), but detailed comparison with the observation is yet to be performed. We find several giant elliptical galaxies instead of a cD galaxy. The origin of the difference between clusters with and without cD galaxies is not yet clear

Results

Discussion

Conclusion