The role of dry mergers for the formation and evolution of brightest
  cluster galaxies
  Using a resimulation technique, we perform high-resolution cosmological
simulations of dry mergers in a massive galaxy cluster identified in the
Millennium Run. Our initial conditions include well resolved compound galaxy
models consisting of dark matter halos and stellar bulges that are used to
replace the most massive cluster progenitor halos at redshift z=3. By
construction, our galaxy models obey the stellar mass-size relation initially.
We demonstrate that the brightest cluster galaxy (BCG) evolves away from the
Kormendy relation as defined by the smaller mass galaxies (i.e., the relation
bends). As a result of the comparatively large number of mergers the BCG
experiences, its total mass-to-light ratio becomes significantly higher than in
typical elliptical galaxies. We also show that the mixing processes between
dark matter and stars lead to a small but numerically robust tilt in the
fundamental plane and that the BCG lies on the tilted plane. Our model is
consistent with the observed steepening of the logarithmic mass-to-light
gradient as a function of the stellar mass. Surprisingly, we find only
tentative weak distortion in the Faber-Jackson relation that depends on the
aperture size, unlike expected based on studies of isolated merger simulations.
This may be due to differences in the distribution of galaxy orbits, which is
given in our approach directly by the cosmological context while it has to be
assumed in isolated merger simulations, and the fact that the BCG is located
deep in the cluster potential well.
