Galaxies get fat and weird along with their black holes

I mentioned earlier that all big galaxies have big black holes in their hearts. I also mentioned that the size of the black hole is related to various features of the galaxies, but that might be a little weird because, after all, galaxies don't live in a vacuum^*. There are other galaxies out there, and they sometimes collide, forming bigger galaxies. So why should there still be a relationship between the black hole and the galaxy it sits in? Well, the idea is that when two galaxies collide and merge, so do their black holes. Complicated physical processes tend to favor the two black holes getting ever closer, until they eat each other and become one somewhat fatter black hole. At the same time, the galaxy formed by the merger around the two black holes also grows. That's the idea, at least. But there's a way to see if that's ...