Astronomers have looked at the nearby globular cluster NGC 6397 and found that instead of a single massive black hole in the core, there are likely to be dozens or even hundreds of smaller black holes at its center.
Holy Kessel Run!
Black holes play an astrophysically important role in the birth and life of galaxies, stars and other objects. We know two flavors of black holes: those of stellar mass, from a few to a few dozen times the mass of a star that are created when massive stars explode, and supermassives of 100,000 to billions sometimes the mass of the Sun residing in the centers of galaxies.
This is a fairly large mass gap between the two. Astronomers think there is a third type, called intermediate-mass black holes (or IMBHs) ranging from 100 to 100,000 solar masses, that fill this gap. The problem is that evidence is scarce. Only a few candidates have been found, even when they break a star to pieces, when they flee from the centers of dwarf galaxies, or even when they form and shake the fabric of space-time.
A place to look for them is in the centers of globular clusters, roughly spherical collections of hundreds of thousands of stars joined together by their mutual gravity. They are usually only a few dozen light-years in diameter, so the stars are very dense.
This means that the stars in these clusters pass very close to each other all the time, and when they do, something interesting happens: the more massive of the two tends to fall closer to the center of the cluster. cluster and the lightest moves outward. Over time, this means that many of the most massive stars are in the core of the cluster.
This can naturally lead to an IMBH in the center of the cluster. A really massive star can fuse with other stars as it descends, and once installed in the center, it can explode creating a decently massive black hole. It then feeds on other stars or black holes when they fall into it, creating an IMBH. Or it is possible that the regular black holes just fall into the center and end up merging, becoming a single IMBH.
On the other hand, it is also possible that the very center of the cluster has many smaller black star-mass holes and other dark objects such as white dwarfs and neutron stars orbiting around, all the results of the stars that have arrived. at the end of his life. – Distributed in a much larger volume of space than an IMBH would occupy.
However, it is difficult to find evidence of this. One way is to look at the orbits of the stars in the cluster. They all orbit in the center of the collection, and if there is a single black hole, their orbits will be slightly different than if, for example, there was a larger, more diffuse collection of smaller black holes.
This requires, however, incredibly accurate measurements of the stars in the cluster, and until recently this was not possible. A couple of astronomers have taken on the task. They observed NGC 6397, a globular in the constellation of Ara. It is the second closest to Earth at a distance of about 7,800 light-years, making stellar motions easier to measure. So is it relaxed, the strange term used by astronomers means that stars that have been around for a long time and many possibilities to interact with others, so that massive stars can fall in the center. They observed the stars using Hubble, Gaia, and the Very Large Telescope to look at how the stars have moved over time and calculate their orbits.
They then performed a lot of statistical simulations of computer models to see what the orbits should look like if there is an IMBH in the center of NGC 6397 in front of a cloud of black holes.
They proved that it is so possible there is an IMBH there, about 500 to 650 times the mass of the Sun. Although its orbital calculations allow it, realistically, although it is unlikely. As black holes melt to form a larger black hole, they explode energy in the form of gravitational waves. This can kick the resulting black hole, acting like a rocket, giving it a fairly high speed. They found that no less than about 1,000 times the mass of the Sun should have received enough energy to come out of the cluster completely.
This leaves a swarm of dark objects like the culprit that shapes the orbits of the stars. Their models indicate that it fits much better. They found that a mass equal to 1-2% of the total mass of the cluster (equivalent to about 1,000-2,000 times the mass of the Sun) distributed by spherical volume about half a light-year would explain the orbital configurations they see in the stars in the cluster.
It’s a tight fit. The closest star to the Sun is Alpha Centauri, 4.37 light-years from us, but it would have a globular nucleus. thousands of stars in this same volume!
They expect that about half of these objects will be black holes of stellar mass, about 4/5 of the rest will be white dwarfs and 1/5 of neutron stars.
This would turn the center of NGC 6397 into the star cemetery, the ghosts of its former self still haunting the core.
This may be the case for many globular clusters, although further observations will be needed. And it leaves us with a weird problem: we know that IMBHs should exist, there’s no real reason to think they shouldn’t have them, and it’s actually hard to find them.
Looks like we can remove NGC 6397 from this list. Fortunately, there is still a whole Universe around us to search. If they are there, it is a good bet that we will find them.