Artwork based on a real Hubble observation of NGC 6397 showing black holes swarming in its core. Credit: ESA/Hubble, N. Bartmann
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Artwork based on a real Hubble observation of NGC 6397 showing black holes swarming in its core. Credit: ESA/Hubble, N. Bartmann

Black holes swarm in the core of a globular cluster

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Feb 12, 2021, 9:00 AM EST (Updated)

Astronomers have looked at the nearby globular cluster NGC 6397 and found that, instead of a single massive black hole in its core, it's likely to have dozens or even hundreds of smaller black holes swarming around in its 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 flavors of black holes: Stellar-mass ones, from a few up to a few dozen times the mass of a star that are created when massive stars explode, and supermassive ones from 100,000 up to billions of times the Sun's mass that reside in the centers of galaxies.

That's a pretty big gap in mass between the two! Astronomers think there's a third kind, called intermediate mass black holes (or IMBHs) that go from around 100 to 100,000 solar masses, that fills that gap. The problem is the evidence for these 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.

A ground-based view shows the globular cluster NGC 6397 in all its glory. Credit:
D. Verschatse / Antilhue Observatory

One place to look for them is in the centers of globular clusters, roughly spherical collections of hundreds of thousands of stars bound together by their mutual gravity. They tend to be only a few dozen light years across, so the stars are very densely packed.

That means stars in these clusters pass by each other pretty closely all the time, and when they do an interesting thing happens: The more massive of the two tends to drop closer to the cluster center, and the lighter one moves outward. Over time, this means a lot of the more massive stars are in the cluster core.

That can naturally lead to an IMBH in the cluster center. A truly massive star might merge with other stars on its way down, and once it settles into the very center it can explode, creating a decently massive black hole. This then feeds on other stars or black holes as they fall into it, creating an IMBH. Or it's possible regular black holes just fall to the center and eventually merge, growing into a single IMBH.

The center of the globular cluster NGC 6397 is a spectacular assortment of hundreds of thousands of stars. Credit: NASA, ESA, and T. Brown and S. Casertano (STScI) 
Acknowledgement: NASA, ESA, and J. Anderson (STScI)

On the other hand, it's also possible that the very center of the cluster has lots of smaller, stellar-mass black holes and other dark objects like white dwarfs and neutron stars orbiting around — all the results of stars having reached the ends of their lives ­— spread out in a volume of space much larger than what an IMBH would occupy.

Finding evidence for this is hard, though. One way is to look at the orbits of the stars in the cluster. They all orbit the center of the collection, and if there's a single black hole there their orbits will be slightly different than if, say, there's a bigger, more diffuse collection of smaller black holes there.

This requires incredibly precise measurements of the stars in the cluster, though, and until recently this wasn't possible. A pair of astronomers has undertaken the task. They looked at NGC 6397, a globular in the constellation of Ara. It's the second closest one to Earth at a distance of about 7,800 light years, so stellar motions are easier to measure. It's also relaxed, the odd term astronomers use to mean that the stars in it have had a long time and lots of chances to interact with other, such that massive stars can fall to 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 to calculate their orbits.

They then ran a bunch of statistical computer models simulations to see what the orbits should look like if there's an IMBH in the center of NGC 6397 versus a cloud of black holes.

They found that it's possible there's an IMBH there, somewhere from roughly 500 – 650 times the mass of the Sun. While their orbital calculations allow for this, realistically speaking though it's unlikely. As black holes merge to form a bigger black hole they blast out energy in the form of gravitational waves. This can give a kick to the resulting black hole, acting like a rocket, giving it a pretty big velocity. They found that anything less than about 1,000 times the Sun's mass should have received enough energy to leave the cluster entirely!

An image of the globular cluster NGC 6397 (left) with a zoom (right) showing artwork depicting black holes swarming in its core. Credit: ESA/Hubble, N. Bartmann

That leaves a swarm of dark objects as the culprit shaping the stars' orbits. Their models indicate this is a much better fit. They found that a mass equaling about 1-2% of the cluster's total mass — equivalent to about 1,000–2,000 times the Sun's mass — spread out over spherical volume about half a light year would explain the orbital configurations they see in the cluster stars.

That's a tight fit. The nearest star to the Sun is Alpha Centauri, 4.37 light years from us, but a globular core would have thousands of stars in that same volume!

They expect that about half of those objects would be stellar mass black holes, with roughly 4/5 of the rest being white dwarfs and 1/5 neutron stars.

This would make the center of NGC 6397 the graveyard of stars, the ghosts of their previous selves still haunting the core.

It's possible that this is the case for many globular clusters, though that will take further observations to ascertain. And it leaves us with a weird problem: We know IMBHs should exist, there's no real reason we can think of that they shouldn't, and yet actually finding them is turning out to be difficult.

It looks like we can scratch NGC 6397 from that list. Happily, there's still a whole Universe around us to search. If they're out there, it's a good bet we'll find them.