4C +37.11

4C +37.11 or Galaxy 0402+379 is a radio galaxy and elliptical galaxy featuring binary supermassive black holes with the least separation of any directly observed binaries, as of 2006. The separation between the two is 24 light-years or 7.3 parsecs, with an orbital period of 30,000 years. The two supermassive black holes, about 750 million light years from earth, have a combined mass of about 15 billion.

Other supermassive binary black hole candidates suggest the smaller separation distances expected as they eventually merge, but have not been confirmed. For example, quasar OJ 287 is inferred to have a binary supermassive black hole pair with an orbital period of 12 years, and thus be much closer together. However these have not been directly measured and additional observation, possibly over extended time periods, is needed.

The eventual collision of the pair, which should stay apart for at least a few million more years, would result in strong gravitational waves.