Merging of a massive black hole binary II

Abstract

In this paper, the second in a series of two, we justify two important assumptions on which the result is based that in course of a galaxy merger the slingshot ejection of bound stars is sufficiently efficient to allow a supermassive black hole binary to merge. A steep cusp with a power law index of 2.5 to 3 is required which is as massive as the binary and surrounds the BHs when the binary becomes hard. This cusp is probably formed when both clusters, surrounding each black hole, merge and combine with the matter funneled into the center. We find this profile to be in agreement with observed post-merger distributions after the cusp has been destroyed. The time dependency we derive for the merger predicts that stalled black holes, if they exist at all, will preferably be found in less than about 0.2 pc distance. To test this prediction we compute the current semimajor axis of 12 candidates of ongoing mergers. We find all binaries unambiguously to be already in the last phase when they decay due to the emission of gravitational waves. Therefore, in striking contradiction with predictions of a depleted loss-cone, the abscence of even a single source in the slingshot phase strongly supports our previous and current results: Binaries merge due to slingshot ejection of stars which have been funneled into the central regions in course of a galaxy collision.