1995A&A...302..649N

Recent X-ray observations have established that collisions between subclusters of galaxies are rather common phenomena. Prompted by such observations, we have performed N-body simulations of two equal-mass subclusters of galaxies, which are going to merge. We first have confirmed that only a part of kinetic energy associated with the relative motion of two subclusters is converted to internal energy of each subcluster at the first encounter. Two subclusters, therefore, once pass through each other, turn around due to gravitational attraction, and finally merge at the second or third encounter. We performed simulations for a variety of dark-matter (DM) distributions, and find that the time scale for washing out the double peak structures after the first encounter strongly depends on the distribution of the density and velocity distributions of the DM. It takes longer when the DM is spatially extended and/or when the velocity distribution of the DM has a Gaussian shape, rather than a uniform distribution. According to our calculation it takes more than 4x10⁹yr after the first encounter until the density contour shows only a single peak. To explain the high fraction of the clusters with substructures among nearby clusters, Richstone et al. (1992; hereafter RLT) required recent(∼10⁹h^–1yr) cluster formation. However, our results show that the timescale for subcluster merging is still uncertain and possibly much longer than the time scale assumed by RLT. Caution should be exercised when concluding that the density of the universe is high by using RLT's method.