Astronomers have looked at the nearby globular star cluster NGC 6397 and found that instead of a single massive black hole in the core, there are likely dozens or even hundreds of smaller black holes swarming in the center.
Holy Kessel Run!
Black holes play an astrophysically important role in the creation and life of galaxies, stars and other objects. We know two flavors of black holes: Stellar mass, from a few to a few dozen times the mass of a star that forms when massive stars explode, and supermassive ones from 100,000 to billions of times the mass of the sun that reside in the centers of galaxies.
That’s a pretty big difference in mass between the two! Astronomers think there is a third kind, called medium-mass black holes (or IMBHs), that go from about 100 to 100,000 solar masses, filling that gap. The problem is that the evidence for this is scarce. Only a few candidates have been found, including when they tear a star to shreds, when they flee from the centers of dwarf galaxies, or even when they form and shake the fabric of spacetime.
One place to look for them is in the centers of globular clusters, roughly spherical assemblies of hundreds of thousands of stars connected by their mutual gravity. They are usually only a few dozen light years across, so the stars are very densely packed.
That means stars in these clusters pass pretty 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 cluster center, and the lighter one moves out. 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 could merge with other stars on its way down, and once it sinks into the center it can explode creating a pretty massive black hole. This then feeds on other stars or black holes as they fall into them, creating an IMBH. Or it is possible that regular black holes just fall to the center and eventually merge and grow into a single IMBH.
On the other hand, it is also possible that the center of the cluster has much smaller black holes with a stellar mass and other dark objects such as white dwarfs and neutron stars orbiting around it – all results from stars that have reached the end of their lives – scattered in a space 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 cluster. They all revolve around the center of the set, and if there is a single black hole their orbits will be slightly different than if there is, say, a larger, more diffuse collection of smaller black holes.
However, this requires incredibly accurate measurements of the stars in the cluster, and until recently this was not possible. A few astronomers have taken on 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 star movements are easier to measure. It is also relax, the strange term astronomers use to mean that the stars in it have had a long time and many opportunities to communicate with others so that massive stars can fall towards the center. They observed the stars with the help of Hubble, Gaia and the Very Large Telescope to see how the stars have moved over time and to calculate their orbits.
They then ran some statistical computer model simulations 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 found it so possible there’s an IMBH there, somewhere between about 500 – 650 times the mass of the sun. While their orbital calculations allow it, it is realistically unlikely. As black holes merge into a larger black hole, they release energy in the form of gravitational waves. This can give a kick to the resulting black hole, which acts like a rocket and gives it quite a high speed. They found that a little less than about 1,000 times the mass of the sun would have received enough energy to completely leave the cluster!
That leaves a swarm of dark objects as the culprit that forms the orbits of the stars. Their models state that this is a much better fit. They found that a mass equal to about 1-2% of the cluster’s total mass – equal to about 1,000-2,000 times the mass of the Sun – spread over the spherical volume of about half a light year would explain the orbital configurations they see in the cluster stars.
That’s a tight fit. The closest star to the sun is Alpha Centauri, 4.37 light-years away, but a spherical core would be thousands of stars in the same volume!
They expect that about half of those objects will be black holes of stellar mass, while about 4/5 of the rest will be white dwarfs and 1/5 neutron stars.
This would make the center of NGC 6397 the graveyard of stars, while the ghosts of their previous selves still haunt the core.
This may be the case for many globular clusters, but that will require further observations to determine. And it leaves us with a weird problem: we know that IMBHs should exist, there’s no real reason we can think of they shouldn’t, and yet it turns out to be hard to actually find them.
It looks like we can scratch NGC 6397 from that list. Fortunately, there is still an entire universe around us to search. If they are there, it is a good bet we will find them.