In this celestial "billiards" game, chaos reigns.
When three black holes are thrown into the disc that surrounds a supermassive black hole, things get very strange, very quickly.
That's the conclusion of new research into a particularly unusual gravitational wave event reported in May 2019 that scientists are still attempting to figure out. Gravitational waves are ripples in space-time created by spectacular events such as black hole mergers, among others. However, this collision differed from others observed by scientists: it produced a black hole in the mid-size range that scientists can barely see, let alone explain, and some force was expanding the typically circular dance as the behemoths neared each other.
"The GW190521 gravitational wave incident is the most unexpected discovery to yet. The masses and spins of the black holes were already remarkable, but what was even more surprising was that they didn't appear to be in a circular orbit leading up to the merger "In a statement, Imre Bartos, a physicist at the University of Florida and co-author on the new study, said. (Gravitational wave signals are given names based on the date they were discovered, therefore GW190521 denotes a gravitational wave identified on May 21, 2019.)
Scientists thought the merger occurred in a pocket of space rich in black holes as early as the first study of the odd signal. There are two forms of black holes known to astronomers. One type, known as stellar black holes, is formed by dying stars and has a mass of a dozen times that of our sun. Supermassive black holes, on the other hand, can be found at the heart of some galaxies (including our own Milky Way) and can hold millions of times the mass of their smaller counterparts.
The consequence of the merging in May 2019 seemed to be an intermediate black hole, with a mass between 100 and 1,000 times that of our sun. Scientists had never been able to investigate or explain how such a thing could form before. Astronomers speculated that one of the merging black holes was the result of a collision, pushing the final product into the enigmatic intermediate region of 142 times the mass of the sun.
The astronomers who studied GW190521 first argued that the event occurred at an active galactic core - a very dynamic supermassive black hole anchoring a galaxy where lesser black holes might grow — in order to have two sequential collisions.
The current study backs up that assertion by taking a different approach to the problem.
These researchers wanted to know why the two black holes didn't actually circle each other as they collided, but instead had eccentric or elliptical orbits that were more ovals than circles. That, too, was strange: astronomers had assumed that the tremendous gravitational forces at work when two black holes collided would compel them to collide in a circular route.
As a result, the researchers behind the new study began modelling black hole encounters. While their calculations predicted that three black holes colliding at random would not result in an eccentric collision, when they considered the surroundings of an active galactic nucleus, something changed.
A disc of matter surrounds the supermassive black hole in this type of structure, similar to a much more massive analogue of the solar system. An active galactic nucleus, like the solar system, includes stellar black holes dispersed throughout the disc in what is basically a two-dimensional structure, according to recent study.
According to co-author Johan Samsing, an astrophysicist at the Niels Bohr Institute in Denmark, the probability of an eccentric merger in the models increased by up to 100 times under certain conditions. At that pace, perhaps half of all mergers in the discs of active galactic nuclei would be eccentric rather than circular, making May 2019's unusual finding less remarkable.
"The typical velocity and density of black holes in these environments is so high that smaller black holes bounce around like billiard balls, and wide circular binaries cannot exist," said co-author Bence Kocsis, an astrophysicist at the University of Oxford in the United Kingdom.
The likelihood of eccentric mergers in their model varies depending on the features of the disc surrounding the supermassive black hole, according to the researchers. The next step, they said, is to find more black hole crashes to study.
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