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In many accounts of galaxy mergers, the prompt merging of their central black holes, if any, is stated seemingly as too obvious to need further explanation.

While I don't dispute that this may indeed be the inevitable and prompt outcome, it doesn't seem self-evident, even if both black holes are near the centre of mass of their respective galaxies (and even if so, who is to say the centres of mass meet rather than in effect forming an orbit and the merged galaxy having a new resultant centre of mass?)

After all, from a distance a black hole is no different gravitationally from any other mass distribution and, even if they end up fairly close, gravitational waves seem too feeble to degrade their mutual orbit in any reasonable time unless they are extremely close (by cosmic standards - a few diameters apart say).

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Intereting question, and you have such a sweet avatar picture :-) –  Dilaton Sep 28 '12 at 10:42

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You're quite correct that two isolated black holes would simply orbit each other until over a very very long timescale gravity wave radiation would cause them to coalesce.

But galactic black holes can change their momentum by flinging stars about. The net result isn't that different to the way energy is dissipated in a viscous liquid. The black holes will interact with the stars around them and lose energy in the process until they merge.

There are lots of videos modelling galaxy mergers on YouTube. I particularly like this one. Note how stars are flung away from the galaxies as they collide.

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A footnote with some other search terms: This is the final-parsec problem, and indeed many feel the solution is in dynamical friction. Though papers are still published on the topic, so some would say the details aren't 100% worked out. –  Chris White Jul 28 at 22:44

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