Merging of a massive binary due to ejection of bound stars - 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 the course of a galaxy merger, the slingshot ejection of bound stars is sufficiently efficient to allow a supermassive black hole binary (BHB) to merge. A steep cusp with a power-law index of 2.5-3 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 BH, merge and combine with the matter funnelled into the centre. 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 BHs, if they exist at all, will preferentially be found at less than ~0.2pc 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 absence of even a single source in the slingshot phase strongly supports our previous and current results: binaries merge due to the slingshot ejection of stars which have been funnelled into the central regions in the course of a galaxy collision.