Are these supermassive black holes on a collision course?

The fusion of black holes and gravitational waves they produce is a scientific guarantee. Astronomers have observed several fusions of black holes, all between black holes of stellar mass less than 100 times the mass of our Sun. But we have never seen a fusion of supermassive black holes, those whose mass is millions or billions of times that of our star. and in fact, astronomers are wondering what would be the probability of such a collision. Now, the discovery of two supermassive black holes that come together could help scientists answer the question of what would happen if they met.

This particular pair, each more massive than 800 million Suns, lies in a galaxy 2.5 billion light-years away. The galaxy itself is a fusion remnant – all that remains after two galaxies, each hosting a supermassive black hole, combined. A team led by Andy Goulding of Princeton University made the discovery using the Hubble Space Telescope and released his discovery on July 10 in The letters of the astrophysical journal.

Although astronomers often discover confusing galaxies, they have never caught a pair of supermassive black holes that clash. And even, "[merging] The binaries of supermassive black holes produce the most powerful gravitational waves in the universe, "said Chiara Mingarelli, co-author of the study, at the Center for Computing Astrophysics of the Flatiron Institute, in a press release. The pairs of supermassive black holes, said Mingarelli, should emit gravitational waves "a million times stronger" than those that LIGO and VIRGO have already heard.

The last problem of parsec

The supermassive black holes fuse a little differently from their lower mass counterparts. Because they start very far apart, in two different galaxies, it takes a long time for the black holes to meet in the middle of a galaxy fusion. Astronomers believe that supermassive black holes slowly fall to the center of the last remaining galaxy and begin to circle around each other.

But there is a problem. Some theories claim that once the supermassive black holes reach a certain distance – 1 parsec, or about 3.2 light-years -, they move apart and stop. This is because, to get closer to each other, black holes give up their energy to other objects, such as gas, dust and stars too close. But as the distance between the black holes decreases, the space available for objects that sink and steal energy also decreases. Astronomers estimate that in about 1 parsec, there is simply not enough room to contain the amount of "substance" required for the black holes to release enough energy to close and collide.

Thus, from there, the black holes gravitate incessantly between them, without getting closer or confused. Astronomers call this the last problem of parsec, because it is difficult to obtain two supermassive black holes with less than one parsec. Some theories claim that it is only in cases where a supermassive third black hole is present, for example when three galaxies fuse, that it is possible to perform a fusion of supermassive black holes.

So, why not look for supermassive black holes at less than 1 pager to see if this can happen? Astronomers would certainly do it if they could, but at a great distance from other galaxies, today's telescopes simply can not separate two objects so close to each other. They look like an object. To add insult to injury, fusion of supermassive black holes does not produce their deafening gravitational waves as long as they are not less than one jump per second at the final approach. Even if this happens, it is rare, so it is not unusual that the gravitational waves of a supermassive fusion have not yet been heard.

"Astronomy is very embarrassed not to know if supermassive black holes come together," said co-author Jenny Greene at Princeton. "For all black hole physics specialists, observation is a long-standing puzzle that we must solve."

What can astronomers do to determine if the final parsec problem is really a problem? If theories are wrong and there is no problem, the pairs of supermassive black holes should approach each other and merge frequently enough to create a "buzzing" background of gravitational waves. "This sound is called gravitational grounding, and it's kind of like a chaotic chorus of crickets singing in the night," Goulding said. "You can not distinguish one locust from another, but the volume of the noise helps you estimate the number of locusts that exist." But this buzz exceeds the audience capabilities of LIGO and VIRGO, although astronomers are eagerly awaiting projects such as LISA. , a space-based gravitational wave detector that "hears" at frequencies that current instruments can not.

Astronomers can also use pulsars – the remains of massive stars that have quickly collapsed to form neutron stars instead of black holes – to capture the passage of gravitational waves. By the way, the gravitational waves stretch and shrink space-time as we go along, which would disturb very little the regular jolts we receive from pulsars. This disruption would amount to modifying the received signal by a few hundred nanoseconds in the last decade or so, but if astronomers could detect such a change, it could also help them determine whether supermassive black holes can merge.