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 M_☉.^[3]

4C +37.11
Observation data (J2000 epoch)
Constellation	Perseus
Right ascension	04h 05m 49.2s[1]
Declination	+38° 03′ 32″[1]
Redshift	16,500 ± 300 km/s[1]
Distance	750 Mly (230 Mpc) [2]
Apparent magnitude (V)	17.2[1]
Characteristics
Type	Sy[1]
Size	~450,400 ly (138.10 kpc) (estimated)[1]
Other designations
GLXY J0405+380, B2 0402+37, 2MASX J04054928+3803320, RX J0405.8+3803, PGC 2820486

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.^[3]