Thanks to the discovery of gravitational waves, we know that stellar-mass black holes – the ones just a bit bigger than the Sun – can collide and merge. But what about the supermassive black holes that live in the center of galaxies? They are millions if not billions of times the mass of our stars and their possible merger is still a complex problem in astronomy.
Researchers have now found two supermassive black holes on a collision course, located in a galaxy 2.5 billion light-years away. As reported in The Astrophysical Journal Letters, the two supermassive black holes each weigh more than 400 million times the mass of the Sun. They are roughly 430 parsecs away from each other and are shooting two enormous plumes of material. They are definitely active.
When galaxies collide, their cores get an influx of material. Some of this is absorbed by the black holes, while some is thrown out, fueling the plume but also allowing for another process to take place. By throwing gas and stars away from the center, supermassive black holes lose energy and end up moving closer and closer to one another.
One would expect that this process will eventually lead to the complete merger of the supermassive black holes, but the jury and the science is still out. Based on how we think the universe works when the black holes get one parse apart (roughly 3.2 light-years), they shouldn’t have more stuff to throw away to get closer.
This is the “final-parsec problem” and researchers want to solve it. An interesting aspect of this is that if supermassive black holes are closer than a parsec, they begin to release extremely powerful gravitational waves.
“Supermassive black hole binaries produce the loudest gravitational waves in the universe. [They] are a million times louder than those detected by LIGO," co-discoverer Chiara Mingarelli, an associate research scientist at the Flatiron Institute's Center for Computational Astrophysics, said in a statement.
The discovery of this binary black hole pair allowed astronomers to estimate that if the final-parsec problem can be overcome, there should be at least 112 nearby supermassive black hole pairs emitting gravitational waves, which is combined in a general noise in the gravitational signals.
"This noise is called the gravitational wave background, and it's a bit like a chaotic chorus of crickets chirping in the night," lead author Andy Goulding explained. "You can't discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there."
The team believes that within five years, it could be possible to measure this gravitational wave background and help clarify just what happens to supermassive black holes when they get too close to each other.