Two-Temperature Intracluster Medium in Merging Clusters of Galaxies

We investigate the evolution of the intracluster medium during a cluster merger, explicitly considering the relaxation process between the ions and electrons using N-body and hydrodynamical simulations. When two subclusters collide, a bow shock is formed between the centers of the two substructures and propagates in both directions along the collision axis. The shock primarily heats the ions because the kinetic energy of an ion entering the shock is larger than that of an electron by the ratio of their masses. In the postshock region, the energy is transported from the ions to the electrons via Coulomb coupling. However, since the energy-exchange timescale depends on both the gas density and the temperature, the distribution of the electron temperature becomes more complex than that of the plasma mean temperature, especially in the expanding phase. After the collision of two subclusters, gas outflow occurs not only along the collision axis but also in its perpendicular direction. The gas originally located in the central parts of the subclusters moves in both the parallel and perpendicular directions. Since the equilibrium timescale of the gas along these directions is relatively short, the temperature difference between ions and electrons is larger in the directions tilted at angles of ±45° with respect to the collision axis. The electron temperature could be significantly lower than the plasma mean temperature, by at most ~50%. The significance of our results for the interpretation of X-ray observations is briefly discussed.