Astronomers looked at the nearby spherical cluster NGC 6397 and found that instead of a single massive black hole in its core, it would probably have dozens or even hundreds of smaller black holes in the center.
Holy Kessel Run!
Black holes play an astrophysically important role in the birth and lives of galaxies, stars and other objects. We know of two scents of black holes: stellar masses, from a few to several dozen times the mass of a star created when massive stars explode, and supermassive masses of 100,000 to billions of times the solar masses that live in the centers of galaxies.
This is a pretty big gap in mass between the two! Astronomers believe there is a third type, called black mass holes (or IMBHs) that range from about 100 to 100,000 solar masses, filling the gap. The problem is that the evidence for this is scarce. Only a few candidates were found, including when they tore a star to shreds, when they fled from the centers of dwarf systems, or even when they formed and shook the tissue of space-time.
One place to look for them is in the centers of spherical clusters, about spherical collections of hundreds of thousands of stars bound together by their mutual seriousness. They tend to be only a few dozen light-years confused, so the stars are very dense.
This means that stars in these clusters pass fairly close to each other all the time, and when this happens, an interesting thing happens: the more massive of the two tend to fall closer to the center of the cluster, and the lighter moves outward. . Over time, this means that many of the more massive stars are in the cluster core.
This can of course lead to an IMBH in the cluster center. A truly massive star can merge with other stars on its way down, and once it settles in the center, it can explode and create a decent massive black hole. It then feeds on other stars or black holes as they fall into it, creating an IMBH. Or it is possible that ordinary black holes fall just to the middle and eventually merge and grow into a single IMBH.
On the other hand, it is also possible that in the middle of the group are much smaller, black mass holes and other dark objects such as white dwarfs and neutron stars – all the results of stars that came at the end of their lives. – distributed in a volume of space that is much larger than what an IMBH would occupy.
However, it is difficult to find evidence for this. One way is to look at the orbits of the stars in the group. They all orbit in the center of the collection, and if there is a single black hole there, their orbits will be slightly different than, for example, there is a larger, diffuser collection of smaller black holes there.
However, it requires incredibly accurate measurements of the stars in the group, and until recently this was not possible. A few astronomers undertook the task. They looked at NGC 6397, a spherical in the constellation of Ara. It is the second closest to Earth at a distance of about 7,800 light-years, so stellar motion is easier to measure. It is also relaxed, the strange term used by astronomers means that the stars in it had a long time and had many opportunities to communicate with others, so that massive stars could fall in the middle. They observed the stars using Hubble, Gaia and the Very Large Telescope to see how the stars moved over time and to calculate their orbits.
Then they performed a number of statistical computer model simulations to see what the orbits should look like if there was an IMBH in the middle of NGC 6397 opposite a cloud of black holes.
They found that it is possible there is an IMBH there, somewhere of about 500 – 650 times the mass of the sun. Although this makes it possible in their orbital calculations, it is realistically unlikely. As black holes merge into a larger black hole, their energy explodes in the form of gravitational waves. It can kick the resulting black hole, act like a rocket and give a fairly large velocity. They found that anything less than about 1000 times the mass of the sun must have received enough energy to leave the group completely!
This leaves a swarm of dark objects behind as the culprit that forms the stars’ orbits. Their models indicate that it fits much better. They found that a mass of about 1-2% of the total mass of the group – equivalent to about 1,000-2,000 times the mass of the sun – is about half a light-year spread over spherical volume, would the orbital configurations that see them in the cluster stars, explain.
It’s a tight pace. The closest star to the sun is Alpha Centauri, 4.37 light-years from us, but would be a spherical core thousands stars in the same bundle!
They expect that about half of these objects would be black mass holes, with about 4/5 of the remaining white dwarfs and 1/5 neutron stars.
This would make the center of NGC 6397 a cemetery of stars, and the ghosts of their former selves still haunt the core.
This is possible for many globular clusters, although further observations are needed to determine. And that leaves us with a strange problem: we know that IMBHs should exist, there is no real reason why we can think that they should not, and yet it is hard to find them.
Looks like we could scratch NGC 6397 off that list. Fortunately, there is still a whole universe to look for us. If they’re out there, it’s a good choice we’ll find.