The point isn't the absolute amount of mass, but how deep the gravitational wells go -- how rapidly objects can orbit. It's not only about massive objects; it's

The point isn't the absolute amount of mass, but how deep the gravitational wells go -- how rapidly objects can orbit. It's not only about massive objects; it's about compact ones. If you have only one supermassive black hole, then the 2nd and 3rd objects are a binary star, and the interaction between them is very weak. Stars can't orbit each other faster than ~100's of km/s (~1,000's for white dwarfs): they're limited by their low density -- they can't get very close or they'll collide. If, instead, you have a star orbiting a black hole -- that's a very dense object, it allows much closer approaches, much deeper into the potential well, with orbit speeds approaching the speed of light. Don't forget this is a three body interaction. You want a star kicked out of a black hole, with much more kinetic energy than it had going in. This kinetic energy comes from the potential energy of some other object falling into the black hole. How strongly the hypervelocity star gets kicked out, depends on how strongly it can interact with the infalling object, to transfer its energy. (This idea is I think implicit in the paper (page 3): the "kick" velocity of a binary object falling into a black hole (v_kick) is at most a small constant times their mutual escape velocity (v_23)). http://arxiv.org/abs/1411.5022 http://arxiv.org/abs/1411.5022