Astronomers looked at the nearby globular cluster NGC 6397 and found that instead of a single massive black hole in its core, there are likely to be dozens or even hundreds of smaller black holes swarming around it.
Saint Kessel Run!
Black holes play an important astrophysical role in the birth and life of galaxies, stars and other objects. We know about two flavors of black holes: those with a stellar mass, from a few to a few dozen times the mass of a star that are created when massive stars explode, and supermassive ones from 100,000 to billions of times the mass of the Sun, which live in the centers of galaxies.
This is a pretty big mass gap between the two! Astronomers believe that there is a third type, called black holes with intermediate mass (or IMBH), which goes from about 100 to 100,000 solar masses, which fills this gap. The problem is that evidence for them is scarce. Only a few candidates have been found, including when they break a star to pieces, when they flee from the center of dwarf galaxies, or even when they form and shake the space-time fabric.
A place to look for them is in the center of globular groups, roughly spherical collections of hundreds of thousands of stars connected by their mutual gravity. They tend to be only a few decades light, so the stars are very dense.
This means that the stars in these groups pass each other pretty much all the time, and when they do something interesting, they happen: The more massive of the two tends to descend closer to the center of the cluster, and the lighter one moves toward exterior. Over time, this means that a lot of more massive stars are in the core of the cluster.
This can naturally lead to an IMBH in the center of the cluster. A truly massive star could merge with other stars as it descends, and once it sits right in the center, it can explode, creating a decently 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 in the center and eventually merge, becoming a single IMBH.
On the other hand, it is also possible that even the center of the cluster has a lot of smaller black holes with stellar mass and other dark objects, such as white dwarfs and neutron stars orbiting around them – all results stars that have reached the end of their lives – stretched in a much larger volume of space than what would occupy an IMBH.
Finding evidence for this, however, is difficult. One way is to look at the orbits of the stars in the group. They all orbit the center of the collection, and if there is a single black hole there, their orbits will be slightly different than if, say, there was a larger and more diffuse collection of smaller black holes there.
This requires incredibly accurate measurements of the stars in the group, however, and until recently this was not possible. A pair of astronomers have taken on the task. They looked at NGC 6397, a globular in the constellation Ara. It is the second closest to Earth at a distance of about 7,800 light-years, so stellar motions are easier to measure. It is also relaxed, the strange term used by astronomers means that the stars in it had a long time and many chances 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 see how the stars moved over time and to calculate their orbits.
Then they ran a bunch of statistical simulations of computer models to see what the orbits should look like if there was an IMBH in the center of NGC 6397 against a cloud of black holes.
They found out it is possible there is an IMBH there, somewhere about 500-650 times the mass of the Sun. Although their orbital calculations allow this, realistically speaking, although it is unlikely. As black holes fuse to form a larger black hole, they release energy in the form of gravitational waves. This can hit the resulting black hole, acting as a rocket, giving it a fairly high speed. They found that something less than about 1,000 times the mass of the Sun should have received enough energy to leave the cluster altogether!
This leaves a swarm of dark objects as the culprit that shapes the orbits of the stars. Their models indicate that it is a much better fit. They found that a mass equal to about 1-2% of the total mass of the cluster – equivalent to about 1,000-2,000 times the mass of the Sun – spread in spherical volume about half a light-year would explain the orbital configurations they make. I see in the cluster stars.
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 core. THOUSAND of stars in the same volume!
They expect about half of these objects to be black stellar holes, about 4/5 of the rest being white dwarfs and 1/5 neutron stars.
This would make the center of NGC 6397 the graveyard of the stars, the ghosts of their former self still haunting the core.
This may be the case for many globular clusters, although this will require further observation to establish. And it leaves us with a strange problem: we know that IMBHs should exist, there is no real reason to think they shouldn’t, and yet finding them is proving difficult.
Looks like we can scratch NGC 6397 from that list. Fortunately, there is still a whole Universe around us to look for. If they’re there, it’s a good bet we’ll find them.