Astronomers looked at The adjacent globular cluster NGC 6397 And they found that, rather than having a single massive black hole at its core, it would likely have tens or even hundreds of smaller black holes clustering at its center.
Black holes play an important astrophysical role in the birth and life of galaxies, stars, and other things. We know two types of black holes: those with stellar mass, from a few to a few tens of star mass that are formed when massive stars explode, and supermassive flavors from 100,000 to billions of times the mass of the Sun reside in the centers of galaxies.
That’s a huge gap in the mass between the two! Astronomers believe that there is a third type, called medium-mass black holes (or IMBHs) of 100 to 100,000 solar masses, that fill this gap. The problem is that evidence for this is scarce. Only a few candidates are found, including When they tear a star to shredsAnd the When they flee from the centers of dwarf galaxiesor Even when they form and vibrate the fabric of space-time.
One place to look for it in centers Spherical groups, Nearly spherical clusters of hundreds of thousands of stars, linked together by their mutual gravity. They tend to be a few tens of light years away, so stars are very crowded.
This means that the stars in these clusters pass closely together all the time, and when they do something interesting happens: the larger the mass of the two tends to drop near the cluster center, and the lighter it moves outwards. Over time, this means that many of the more massive stars are at the core of the cluster.
This can naturally cause IMBH to be at the center of mass. A truly massive star might merge with other stars on its way down, and once it settles in the same center, it could explode, resulting in a supermassive black hole. This then feeds on stars or other black holes as they fall into them, giving rise to the IMBH formation. Or, a regular black hole could fall in the center and eventually merge and grow into a single hole.
On the other hand, it is also possible that the center of the cluster contains a lot of small stellar mass black holes and other dark objects such as White dwarves And the Neutron stars They are orbiting – all the results of stars reaching the end of their lives – scattered over a volume of space much larger than IMBH occupies.
Evidence for this is difficult to find. One way is to look at the orbits of stars in a cluster. They all orbit around the center of the cluster, and if there was one black hole there, their orbits would be slightly different than if there were, for example, a larger and more diffuse cluster of smaller black holes there.
This required incredibly accurate measurements of the stars in the cluster, however, this wasn’t possible until recently. A pair of astronomers took over this task. They looked at NGC 6397, which is a spherical shape in constellation Ara. It is the second closest point to Earth at a distance of about 7,800 light-years, so stellar motions are easy to measure. it’s also relaxation, The strange term that astronomers use to denote that stars in it have passed a long time and many opportunities to interact with others, such as 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 moved over time, and to calculate their orbits.
Then they ran a set of statistical computer simulations to see what the orbits would look like if there were IMBH in the center of NGC 6397 against a cloud of black holes.
They found it Maybe There’s an IMBH out there, somewhere roughly 500-650 times the mass of the Sun. While their orbital calculations allow for this, realistically though it is unlikely. When black holes merge to form a larger black hole, they release energy in the form of gravitational waves. This could give a boost to the resulting black hole, behave like a missile, and give it a very high velocity. And they found that anything less than 1,000 times the mass of the Sun must have received enough energy to leave the mass entirely!
This leaves a swarm of dark objects as the culprit forms the orbits of the stars. Their models indicate this is a lot better. They found that a mass equal to about 1-2% of the cluster’s total mass – roughly 1,000-2,000 times the mass of the Sun – spread over a spherical size of about half a light-year would explain the orbital formations they see in cluster stars.
This is a tight fit. The closest star to the sun Alpha Centauri, 4.37 light-years away from us, but a globular core would be In thousands Of stars the same size!
They expect that half of these objects will be stellar-mass black holes, with approximately 4/5 of the remainder being white dwarfs and 1/5 of neutron stars.
This would make NGC 6397 a graveyard of stars, and the ghosts of their former selves still haunt her heart.
This is likely to be the case for many globular clusters, although this will require more observations to be certain. This leaves us with a strange problem: We know IMBHs should exist, there’s no real reason we can think they shouldn’t, and yet finding them is actually difficult.
Looks like we can delete NGC 6397 from that list. Thankfully, there’s still an entire universe around to look into. If they are there, it is a good idea to find them.