A team of astrophysicists in India has discovered an unusual merger of three supermassive black holes. The three merging black holes were all found in galaxies in the constellation Toucan. Data from the Ultraviolet Imaging Telescope (UVIT) onboard ASTROSAT, the European integral field optical telescope MUSE installed on the Very Large Telescope (VLT) in Chile, and infrared pictures from the optical telescope (IRSF) in South Africa were used to make the finding.
The final parsec issue is solved by the presence of a third black hole. When two galaxies collide, the kinetic energy in their black holes is transferred to the surrounding gas, bringing them closer together. The distance between the blackholes shrinks with time until it is less than a parsec (3.26 light-years). The two black holes will thereafter be unable to lose any more kinetic energy and will combine. The last parsec issue is what it’s called.
When another black hole or a star passes by and takes away part of their total angular momentum, the two black holes can become closer. In the presence of a third blackhole, the dual merging blackholes merge with each other.
Source: The Hindu