While the earlier conclusions on BCG evolution held up, the new simulations showed that the central mass re-distributes itself significantly as mergers proceed. By the present day, the mixture of dark and stellar matter in the BCGs had the same total mass density profiles as in test simulations which included dark matter alone. This demonstrated that evolution tends to drive the total mass density profile (stars and dark matter) towards the "universal" shape. Since the stars contribute most of the mass near the middle of the final BCGs, this meant that their dark matter density profiles were actually less centrally concentrated than in the dark-matter-only simulations, even though they started out more concentrated in the initial galaxies. As a result, the simulated BCGs appear to have dark matter profiles consistent with those inferred observationally.
The simulated BCGs typically experienced 6 or 7 mergers which, in real galaxies, would be accompanied by a merger of the central supermassive black holes. Such mergers pump energy into the innermost regions, causing the stars and dark matter to move outwards. Estimates of the size of this effect based on the simulations suggest that it might explain the large stellar cores often observed in BCGs. So far, the effects of supermassive black holes in BCGs cannot be directly simulated in a full cosmological context, so the current simulations offer realistic initial conditions for simplified numerical studies of supermassive black hole merging in the central regions of BCGs.
This study suggests that observations of the mass distribution in the centres of galaxy clusters can be understood if BCG evolution is primarily driven by dissipationless mergers. Within the standard LCDM paradigm, such an evolutionary path naturally explains a total density profile similar to those found in dark-matter-only simulations, together with a shallower dark matter density profile. There seems no need to appeal to the more radical explanations proposed in some recent papers such as new physics in the dark matter sector or dynamical effects driven by star and black hole formation which are much more violent than any observed.
Chervin Laporte and Simon White