Clusters of galaxies contain not only galaxies but also X-ray emitting hot intracluster medium (ICM) with the temperature of 10**(7-8)K, whose mass is comparable to or larger than that of galaxies. In the ICM, a large amount of metals exists, suggesting that significant fraction of the ICM is metal-rich processed gas ejected from member galaxies. Therefore, by investigating metal abundances of the ICM, we can obtain information about the origin of such a large amount of metals; star formation history in galaxies and clusters. Before ASCA launch, only emission-weighted Fe abundance was available, which is easily affected by abundance gradients. Moreover, the abundance ratios between alpha-elements and Fe are important information about the origin of metals in the ICM, as well as the abundance distribution. Utilizing the unprecedented X-ray spectroscopic capability of ASCA together with its moderate imaging capability, we measured metal abundances of the ICM for nearby 40 clusters of galaxies with various ICM temperatures.
We obtained spatially averaged metal abundances of not only Fe but also Si and S as a function of the ICM temperature, free from the affections of abundance gradients. The Fe and S abundances are 0.2-0.3 solar for any ICM temperature. In contrast, the Si abundance increases from 0.3 to 0.6-0.7 solar as the ICM temperature increases. As a result, the Si to Fe abundance ratio correlates positively with the ICM temperature. The Si-rich metal abundance in rich clusters and the reduction of the Si to Fe abundance ratio in poor clusters require both type II and Ia supernovae (SNe II and SNe Ia) significantly in the metal enrichment of the ICM; 80% of Si and 30-60% of Fe in rich clusters came from SNe II. The contribution of SNe Ia to the ICM is for the first time confirmed. Within the density limited radii, the metal to stellar mass ratio has been found to decrease towards cooler clusters, especially for Si. We suggest that poor clusters could not confine significant amount of metals from SNe II, although other possibilities cannot be ruled out. The following scenario of chemical evolution is implied; violent SNe II driven galactic wind in the early galaxy formation for alpha-elements enrichment, and prolonged SNe Ia contribution for Fe enrichment.
We found that metal abundance increase is a common phenomenon around the cD galaxies. These excess metals are likely to be metal-eich ejectors from cD galaxies. The solar-like abundance ratio around cD galaxies indicates that excess metals around cD galaxies came from SNe Ia. This again needs the contribution of SNe Ia in the ICM enrichment.