  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. 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. According to our calculation it takes more than $4 imes
10^9$yr 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,Loeb Turner(1992; hereafter RLT) required recent($$ $10^9
h^{-1}$yr) 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.
