Off-center mergers of clusters of galaxies and nonequipartition of electrons and ions in the intracluster medium

We investigate the dynamical evolution of clusters of galaxies and their ob servational consequences during off-center mergers, explicitly considering the relaxation process between ions and electrons in the intracluster mediu m by N-body and hydrodynamical simulations. In the contracting phase, a bow shock is formed between the two subclusters. The observed temperature betw een two peaks in this phase depends on the viewing angle, even if the geome try of the system seems to be very simple like head-on collisions. Around t he most contracting epoch, when we observe merging clusters nearly along th e collision axis, they look like spherical relaxed clusters with large temp erature gradients. In the expanding phase, spiral bow shocks occur. As in h ead-on mergers, the electron temperature is significantly lower than the pl asma mean one, especially in the postshock regions in the expanding phase. When the systems have relatively large angular momentum, double-peak struct ures in the X-ray images can survive even after the most contracting epoch. Morphological features in both X-ray images and electron temperature distr ibution characteristic to off-center mergers are seriously affected by. the viewing angle. When the clusters are observed nearly along the collision a xis, the distribution of galaxies' line-of-sight (LOS) velocities is a good indicator of mergers. In the contracting phase, a negative kurtosis and a large skewness are expected for nearly equal mass collisions and rather dif ferent mass ones, respectively. To obtain statistically significant results , about 1000 galaxies' LOS velocities are required. For nearby clusters (z < 0.05), large redshift surveys such as the Two-Degree Field will enable us to study merger dynamics.