Coma cluster: Hot and cool mixture

ASCA has revealed a rather complex temperature structure in the intracluster medium (ICM) around the Coma cluster. Significantly, hot parts (with kT greater than or equal to 10 keV) are located far (similar to 1 Mpc) distant from both the center of the Coma cluster and the galaxy group NGC 4839. There also exist relatively cool regions (with kT less than or similar to, 5 keV). Using N-body + Hydrodynamic simulations, we have demonstrated that such an observed temperature distribution is naturally accounted for if the subgroup NGC 4839 passed through the center of the Coma cluster about 1 Gyr ago, which was first proposed by Burns et al. When the subcluster passed through the Coma cluster, the ICM in the subcluster was compressed due to the ram pressure by the ICM in the Coma, and there formed a bow shock with an are shape just between the two centers. The ICM has thus been heated there, emitting hard radiation. A part of the ICM is reflected by the shock front, and flows backward (with respect to the direction of the subcluster motion) around the Coma cluster center, being cooled due to an adiabatic expansion. After detouring the center, cooled gas collides with each other at the opposite side of the Coma cluster (to the place of the subcluster): producing another hot region. These simulated features are in good agreement with what ASCA found. Further, we present rather unique features in the temperature profiles that can be caused by a merger (or an encounter) with a subgroup of galaxies.