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We present cosmological N-body resimulations of the assembly of the BrightestCluster Galaxies (BCGs) in rich clusters. At $z=2$ we populate dark mattersubhalos with self-gravitating stellar systems whose abundance and structurematch observed high-redshift galaxies. By $z=0$, mergers have built much largergalaxies at cluster centre. Their dark matter density profiles are shallowerthan in corresponding dark-matter-only simulations, but their total massdensity profiles (stars + dark matter) are quite similar. Differences are foundonly at radii where the effects of central black holes may be significant. Darkmatter 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 supportearlier suggestions that NFW-like profiles are an attractor for thehierarchical growth of structure in collisionless systems -- total mass densityprofiles asymptote to the solution found in dark-matter-only simulations overthe radial range where mergers produce significant mixing between stars anddark matter. Simulated dark matter fractions are substantially higher in BCGsthan in field ellipticals, reaching 80\% within the half-light radius. We alsoestimate that supermassive black hole mergers should create BCG cores as largeas $r_{c}\sim 3 \,\mathrm{kpc}$. The good agreement of all these propertieswith recent observational studies of BCG structure suggests that dissipationalprocesses have not played a dominant role in the assembly of the observedsystems.

The re-distribution of matter in the cores of galaxy clusters