  Within the cosmic framework clusters of galaxies are relatively young
objects. Many of them have recently experienced major mergers. Here we
investigate an equal mass merging event at z = ~0.6 resulting in a dark matter
haloe of ~2.2 times 10^{14} Msol/h at z=0. The merging process is covered by
270 outputs of a high resolution cosmological N-body simulation performed with
the ART (adaptive refinement tree) code. Some 2 Gyrs elapse between the first
peri-centre passage of the progenitor cores and their final coalescence. During
that phase the cores experience six peri-centre passages with minimal distances
declining from ~30 to ~2 kpc/h. The time intervals between the peri-centre
passages continuously decrease from 9 to 1 times 10^8 yrs. We follow the mean
density, the velocity dispersion and the entropy of the two progenitors within
a set of fixed proper radii (25, 50, 100, 250, 500, 1000 kpc/h). During the
peri-centre passages we find sharp peaks of the mean densities within these
radii, which exceed the sum of the corresponding progenitor densities. In
addition to the intermixing of the merging haloes, the densities increase due
to contraction caused by the momentary deepening of the potential well. At the
end of the oscillatory relaxation phase the material originating from the less
concentrated of the two equal mass progenitors is deposited at larger radii and
shows a slightly more radially anisotropic velocity dispersion compared to the
material coming from the more concentrated progenitor. Every peri-centre
passage is accompanied by a substantial drop of the central potential well. We
briefly discuss the possibility that AGN outbursts are triggered by the
periodically changing potential.

Astrophysics