Hydrodynamic Simulations of Merging Galaxy Clusters: Non-Equilibrium Ionization State and Two-Temperature Structure

Abstract

We investigate a non-equilibrium ionization state and an electron–ion two-temperature structure of the intracluster medium (ICM) in merging galaxy clusters using a series of $N$-body and hydrodynamic simulations. Mergers with various sets of mass ratios and impact parameters are systematically investigated, and it is found that, in most cases, ICM significantly departs from the ionization equilibrium state at the shock layers with a Mach number of $\sim $1.5–2.0 in the outskirts of the clusters, and the shock layers with a Mach number of $\sim $2–4 in front of the ICM cores. Accordingly, the intensity ratio between FeXXV and FeXXVI K$\alpha$ line emissions is significantly altered from that in the ionization equilibrium state. If the effect of the two-temperature structure of ICM is incorporated, the electron temperature is $\sim $10%–20% and $\sim $30%–50% lower than the mean temperature of ICM at the shock layers in the outskirts and in front of the ICM cores, respectively, and the deviation from the ionization equilibrium state becomes larger. We also address the dependence of the intensity ratio on the viewing angle with respect to the merging plane.