How big are supermassive black holes formed from the collapse of dense star clusters?

If a star cluster becomes sufficiently relativistic in its center, it is unstable to catastrophic collapse to a black hole. In general, the final state consists of a central massive black hole surrounded by a halo of orbiting stars. The authors present a method for estimating how much mass goes into the black hole and how much is left in the halo following collapse. They find that extreme core-halo clusters, such as those arising from the gravothermal catastrophe, can produce black holes with masses substantially larger than the core mass. This may be crucial for explaining the birth of quasars and active galactic nuclei.