The redistribution of matter in the cores of galaxy clusters

We present cosmological N-body resimulations of the assembly of the Brightest Cluster Galaxies (BCGs) in rich clusters. At z = 2, we populate dark matter subhaloes with self-gravitating stellar systems whose abundance and structure match observed high-redshift galaxies. By z = 0, mergers have built much larger galaxies at cluster centre. Their dark matter density profiles are shallower than in corresponding dark-matter-only simulations, but their total mass density profiles (stars + dark matter) are quite similar. Differences are found only at radii where the effects of central black holes may be significant. Dark matter density slopes shallower than gamma = 1.0 occur for r/r(200) < 0.015, close to the half-light radii of the BCGs. Our experiments support earlier suggestions that NFW-like profiles are an attractor for the hierarchical growth of structure in collisionless systems - total mass density profiles asymptote to the solution found in dark-matter-only simulations over the radial range where mergers produce significant mixing between stars and dark matter. Simulated dark matter fractions are substantially higher in BCGs than in field ellipticals, reaching 80 per cent within the half-light radius. We also estimate that supermassive black hole mergers should create BCG cores as large as r(c) similar to 3 kpc. The good agreement of all these properties with recent observational studies of BCG structure suggests that dissipational processes have not played a dominant role in the assembly of the observed systems.