Black holes collision course — A pair of supermassive black holes on a collision course has been detected. This binary black hole could help researchers solve a mystery: discover if two black holes can merge into one.
Can supermassive black holes merge? Perhaps scientists could finally answer this question with a new discovery. Two huge black holes that seem to be on a collision course have been detected, reports a team of researchers in the journal Astrophysical Journal Letters on July 10, 2019. Their study is available in full on the arXiv platform. org.
If these two giants collide, they could form a “black hole hundreds of times larger than the one located in the center of our own galaxy,” says Kris Pardo, a Ph.D. student in astrophysics at Princeton University and co-author of the study, in a statement. These black holes have a mass equivalent to more than 800 million times that of our Sun. For comparison, Sagittarius A*, the black hole that is most likely in the heart of the Milky Way, is about 4 million solar masses.
These two celestial objects are in the middle of a galaxy located 2.5 billion light-years away from Earth. This means that when we look in their direction, what we see is in the past of the universe — a universe 2.5 billion years younger than ours.
This is also the length of time that researchers believe these black holes need to emit very powerful gravitational waves. They are producing it now, but it would take billions of years for them to come to us.
Nevertheless, the detection of these black holes is important: it could help solve a fundamental puzzle about black holes. At present, the scientific community suspects that all major galaxies have a supermassive black hole in their center. When these huge galaxies collide, scientists assume that their black holes meet and get into orbit around each other, closer and closer.
However, it is still unclear whether these pairs of supermassive black holes eventually merge or come together without ever mixing. One theory assumes that black holes would stagnate when they are at about 1 parsec, or 3.2 light-years away: this slowdown is called the “final parsec problem.” Attempting to observe pairs of blocked black holes in this situation is not possible: they are much too close together for us to see that they are two different objects. The two new black holes detected were 430 parsecs apart 2,5 billion years ago.
What is certain is that the gravitational waves caused by pairs of supermassive black holes are “the strongest in the universe,” says Chiara Mingarelli, an astrophysicist specializing in these waves, a researcher at the Flariton Institute in New York, and author of the study. If the problem of the final parsec is solved, one could very well imagine that many pairs of black holes are currently merging in the universe. The noise emitted by these processes would form a “gravitational wave background”. And if two supermassive black holes merge, a particularly powerful wave would be emitted.
These two black holes were detected using the Hubble Space Telescope. The instrument does not allow to see the black holes (they are optically invisible objects in astronomy), but it was used to discover the luminous stars and the gas attracted by the center of the galaxy. The researchers also saw that this nucleus was throwing gas and deduced that a pair of supermassive black holes was in its center.
Thanks to this observation of a pair of supermassive black holes, scientists can better estimate how many fusions of these huge black holes could potentially be detected in the vicinity of the Earth. In the best case, up to 112 supermassive black holes would emit gravitational waves. Within 5 years, scientists think we could observe a gravitational wave, caused by the fusion of two supermassive black holes. If this is not the case, it could prove that the problem of the final parsec has no solution.
In the meantime, scientists should continue to look for other examples of these gigantic binary black holes.