Clusters of galaxies are the largest gravitationally-bound ensembles in the universe, which consist of hundreds of galaxies, intracluster medium (ICM) and dark matter. From the velocity dispersion of member galaxies and the plasma temperature of the ICM, it has been clear that the mass of dark matter is 70-90% of the total mass. X-ray observations of the ICM are essential for knowing the dynamical evolution of clusters. The dynamical state of clusters of galaxies is determined by the X-ray surface brightness and temperature distribution in the ICM. The X-ray surface brightness distributions were previously reported for many clusters, while there were limited observations for the temperature distribution within a cluster.
The Japanese X-ray astronomy satellite, ASCA, allows us to obtain X-ray images and spectra with high energy resolution in the 0.5-10 keV region. However, its X-ray Telescope (XRT) has a complex Point Spread Function (PSF), depending on the incident X-ray photon energy and the off-axis angle from the optical axis of the XRT. Based on the ASCA full simulation tool, we developed analysis software which takes into account the effect of the complex PSF.
Using this method, we analyzed three nearby rich clusters, Abell 1656(Coma), the Ophiuchus cluster and Abell 2319, observed with ASCA and obtained 2-dimensional temperature distributions and surface brightness distributions in the 0.5-10 keV band. Since these clusters have spatially averaged temperatures of 9-10 keV and relatively symmetric surface brightness distributions, these have been considered to be dynamically well-relaxed clusters. However we found for the first time that these clusters have an extended hot region(> 1 Mpc) with a plasma temperature of 14 keV at its periphery. There also exists a cool region in the Coma cluster with a temperature of 5 keV and a size of 400x800 kpc. We also found the image excess from symmetric distribution, which coincides with the cool region without any relation to the hot region.
These facts directly suggest that these clusters are not well-relaxed dynamically, but that they have recently been formed by the experience of cluster-subcluster merging.
An abundance distribution was obtained for the Ophiuchus cluster whose high abundance regions are mostly seen in the hot region. This cluster has 1.5 times higher averaged abundance than the other two clusters and a more concentrated surface brightness profile. Therefore we conclude that this cluster is dynamically and chemically more evolved than the other two clusters.