Where two distant galaxies collide, three supermassive black holes engage in a gravitational dance. Two of the black holes embrace in a tight orbit, spinning out jets of gas, while the third waits off to the side. Observations of the trio demonstrate that swirling jets can help astronomers find hidden black hole pairs. The finding also suggests that these pairs may be more common than previously thought. Every large galaxy appears to harbor a central supermassive black hole, a pileup of the corpses of huge stars. These black holes, which can weigh well over a billion suns, appear to build up over time from collisions between galaxies.
As two galaxies merge, their central black holes find one another, spiral together and eventually combine into one giant black hole.