# Why is it difficult for 2 black holes to merge?

Apparently as part of the merger of galaxies, sometimes the central supermassive black holes of colliding galaxies get really close, however once they come to a distance of about <1 parsec, it seems that various forces conspire to keep the supermassive holes apart for the most part (see final parsec problem).

Is that true, and if so, what accounts for it? I assume it has something to do with orbital momentum, but I would have still expected the two to rather naturally merge...

• To shed the required amount of angular momentum a lot of additional mass is needed that has to be threaded trough what is probably the eye of a needly in astronomical terms... just waving a giant black foam hand here... I am actually surprised that these things seem to happen at all. – CuriousOne Feb 8 '16 at 23:24
• Serban, please give a source for this claim – Mitchell Porter Feb 8 '16 at 23:56
• Yes I would like to see a source for this claim also thanks – Bill Alsept Feb 9 '16 at 0:29
• It's hard to answer this directly, but the end answer will be "gravitational radiation is how the black holes fall in, and gravitational radiation is weak, because it couples with $G$ and the leading term is the quadrupole term" – Jerry Schirmer Feb 9 '16 at 0:32
• How to think about the magnetic fields from both BH? – HolgerFiedler Feb 9 '16 at 6:08