So what happens to the black hole that supposedly powers the centers of spiral galaxies?
Annihilation?
The black holes reestablish themselves at the center of the combined galaxy and merge into a single larger one. For a while it becomes an active nucleus as it gobbles up nebulae and star clusters, but eventually settles down as it clears its immediate neighborhood. There’s nothing really violent about the merging of black holes, since nothing can escape them; their event horizons just swallow each other. Well almost nothing—merging compact objects are supposed to release a lot of gravity waves according to Einstein’s general relativity. But so far we haven’t been able to find any with our current detectors.