Abstract
Given that a supermassive black hole exists in every bulge galaxy, when these interact and coalesce through dynamical friction, the respective black holes are expected to merge, with the temporary formation of a black hole binary. Theoretically, black hole merging is difficult at close distances. At long distances, the two black holes lose their orbital energy, through dynamic friction on the stars in the bulge of the host galaxies. Then the stars are all ejected and, theoretically, we'd have to wait for a relaxation time (very long, as the stars are virtually collisionless) to find stars that could help the black holes lose energy. In reality, gas intervenes at this stage, accelerating the process. In the final stage, the black holes are so close that relativistic effects come into play, they emit gravitational waves and close in very fast. The problem is therefore restricted to distances of less than a parsec and time scales of less than 100 million years. The problem has been tackled by high-performance numerical simulations with high spatial resolution.
On the observational side, the detection of binary black holes is very rare. This tends to prove that fusion efficiency is higher than expected. Examples of direct evidence can be counted on the fingers of one hand, but there is also plenty of indirect evidence, in the reorientation of radio jets, or the observation of a large number of binary AGNs. At high redshift, supermassive black holes are observed in large numbers, and we should be able to observe intermediate-mass black holes, which are still too rare.
