“Forbidden” black hole merger stuns astrophysicists
When two black holes meet, they circle each other in a kind of gravitational tango, then their event horizons respective end up touching: this is what astrophysicists call coalescence and fusion. A bigger black hole is born, but in this case, 2 solar masses + 3 solar masses = 4 solar masses!
Indeed, the merged black hole then has a mass lower than the other two combined, the “lost” mass being converted into gravitational waves that we have been able to perceive for almost 10 years.
On May 21, 2019, the LIGO (Washington) and VIRGO (Italy) detectors detected a very, very powerful signal, the largest merger of stellar black holes ever captured. A black hole of 66 solar masses mated with another of 85 solar masses, creating a monster of 142 solar masses! All records were broken, but scientists were stunned, because they considered this cosmic episode impossible.
Such a stellar black hole should not exist!
Remember that the black holes whose merger we know how to detect by gravitational waves have nothing to do with those of millions or billions of solar masses which reside at the centers of galaxies. These are so-called supermassive black holes and we still don't know how they form.
The black holes in question here are the most common and we know them very well. They are called stellar, meaning that to form them, the collapse of a star with a mass of at least eight times that of our Sun is necessary. At the point of death, the heart of the star is loaded with ultra-heavy elements such as iron and the latter explodes in a supernova, except for the heart itself which collapses under its own mass and becomes a hole. black.
But a theoretical problem arose on May 21, 2019. Indeed, very, very large stars of more than 65 solar masses do not explode like the others and should not leave a black hole! Without going into too much detail, they produce antimatter and decay into what is called a supernova by pair instability. In short, no black hole.
These black holes can only be born near a galactic center…
If we want to be a little open-minded and consider that the first black hole of 66 solar masses was at the limit of theory, then that's fine for him. But the second, that of 85 solar masses, was right in the theoretically prohibited range. It should therefore already be considered as the fusion product of other black holes.
This is what the researchers did the study in questionwho wanted to determine the characteristics of these ancestors. However, they realized that such massive black holes would have given a very significant recoil speed (a “kick”), and the only way to explain why they did not eject each other is to place near a galactic center, where the intense density of matter would hold them.
These overly massive black holes would therefore have formed near a galactic core. So, mystery solved? Not really… Until now, astrophysicists thought that the only place where black holes of around 100 to 10,000 solar masses could form was within globular clusters like this one:
These intermediate mass black holes are definitely very, very mysterious. And as often, if not always, from an observation are born multiple – and exciting – new questions…
