Most all galaxies seem to contain central supermassive black holes.
All too often, galaxies collide (the universe is surprisingly crowded on these scales).
The central black holes may then become bound and merge, which is interesting, or they may hang up in orbit for long enough that another galaxy, and central black hole come crashing the party.
When this happen, things get interesting
The NWU crowd have fun simulating outcomes, this is a topic that has been touched upon before but is seriously under-explored.
PS: the images above is one my own, I didn't like their press release figure... same-same
Don't forget the paper: astro-ph/0612517
I would recommend the work of Manuela Zoccalli of the Pontificial Catholic University of Chile. Her team's observations show that stars in the galaxy's central bulge have a different chemical composition than stars in the spiral arms. If the Milky Way had formed from collisions, stars of similar composition would be mixed together. ESO Press Release Zoccalli's observations also show that our galaxy's central bulge formed in a remarkably short time, less than a billion years. Observations of reality cast doubt on the merger idea.