Paper detail

Are black holes big enough to quench cooling in cluster cool cores?

Total energy arguments (e.g., Fabian et al. 2002) suggest that black holes need to have masses significantly in excess of the prediction from the classic black hole mass - velocity dispersion relation (M-sigma) in order to offset the cooling losses in massive cool core clusters. This suggests that the black holes may be too small to power such clusters. However, Lauer et al. (2007) argue that the black hole mass - bulge luminosity relationship is a better predictor of black hole masses in high luminosity galaxies and that this relationship predicts significantly higher masses in BCGs. They find slow increase in the velocity dispersion with luminosity and a more rapid increase in effective radii with luminosity seen in BCGs as opposed to less luminous galaxies. Motivated by these results and the theoretical work of Boylan-Kolchin et al. (2006) on isolated mergers, we perform high-resolution cosmological simulations of dry mergers in a massive galaxy cluster identified in the Millennium Run including both the dark matter halos and stellar bulges of merging galaxies. We demonstrate that the BCG clearly evolves away from the size-luminosity relation as defined by the smaller galaxies (i.e., the relation bends) and we also see a bending in the luminosity-sigma relation. As black hole mass is expected to be proportional to the mass and luminosity of the stellar bulge of the BCGs (if they were formed in predominantly dissipationless mergers), our findings are consistent with those of Lauer et al. (2007) on a qualitative level and suggest that the black holes in BCGs may indeed be more massive than predicted from the standard M-sigma relation.