Amid a glob of stars drifting through the Milky Way lurks a monster.
Located about 6,000 light-years away, a globular cluster known as Messier 4 appears to be clustered around a black hole about 800 times the mass of our Sun.
That’s not a featherweight, but it’s far from a huge one either. In fact, the object falls into an intermediate mass range that is rarely seen, between much smaller black hole and supermassive chonkers.
So far, their only detections between black holes are often indirect and inconclusive, and this one is no exception.
It is, however, one of the best candidates yet, and soon follow-up studies can easily be performed. This could help us finally find one of these elusive objects, and solve one of the black hole’s most perplexing mysteries.
“Science is rarely about discovering something new in a moment,” says astronomer Timo Prusti of the European Space Agency. “It’s about being more specific in a conclusion step by step, and this could be a step toward being sure that intermediate-mass black holes exist.”
We’ve detected a huge number of black holes in the Universe, and there’s something strange their mass distribution. There are two distinct populations: stellar-mass black holes, up to about 100 times the mass of the Sun; and supermassive black holes, which lie at the center of galaxies and clock millions to billions of Suns.
Between these two sets of mass is… a whole lot of not so much, really. It makes for a great puzzle, which is common, why not? Aren’t there just intermediate mass black holes out there? Or are they out there, and we can’t see them for some reason?
We know how stellar mass black holes are formed – the primary collapse of massive stars, and mergers between these objects. But we’re not too sure about that formation of supermassive black holes. Do they grow from successive mergers of smaller black holes or do they absorb material and swell in size?
Intermediate mass black holes would be a clue, suggesting that they can start small and grow over time. It would certainly make a lot of sense, but the lack of them is a pretty effective barrier to this idea.
A possible location where these black holes may be hidden is in the heart of globular clusters. They are incredibly dense, remarkably spherical clusters of about 100,000 to 1 million or more stars, most of which formed at the same time from the same cloud of gas. Previous studies focusing on globular clusters have found high concentrations of mass at their centers consistent with the mass ranges of intermediate mass black holes.
Messier 4 is the closest globular cluster to Earth. Led by astronomer Eduardo Vitral of the Space Telescope Science Institute, a team of researchers used two powerful space telescopes, Hubble and Gaia, to take a closer look at the stars within. They tracked the movements of about 6,000 stars in the cluster, to see if they could relate those movements to orbits around a small, dense mass.
We wouldn’t normally see black holes if they weren’t actively accreting matter, but those orbits would produce a pretty reliable sign. And their calculations revealed something, with a mass of about 800 solar masses. Even what that something might be is not clear.
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“Using the latest Gaia and Hubble data, it is not possible to distinguish between a dark population of stellar remnants and a single larger point-like source,” Vitral said. “So one of the possible theories is that instead of being many separate small dark objects, this dark mass could be a medium-sized black hole.”
To try to narrow it down, the team carried out modelling, removing stars to see how this changes the shape of the mass. The ejection of a particularly fast-moving star spreads the mass over a greater distance, as you can see from a host of smaller black holes and neutron stars. Further modeling showed that the mass was not spread over a large enough region of space to form a swarm.
In addition, a lot of black holes will be close together which will really create chaos. Gravitational interactions will send stars flying out of the cluster, smearing it messy beyond the sky. We may, in fact, have seen its effects on a star cluster named Palomar 5.
“We have good confidence that we have a very small region with a lot of concentrated mass. It’s about three times smaller than the densest dark mass we’ve seen before in other globular clusters,” Vitral said.
“Although we cannot absolutely prove that it is a central point of gravity, we can show that it is very small. Too small for us to explain other than being a single black hole. Alternatively, there could be a star. mechanism we don’t know, even with current physics.”
So, barring new physics or unseen stars, an intermediate mass black hole appears to be the most likely explanation at the moment. However, a population of smaller black holes is a plausible explanation. The researchers advise further observations of the cluster using Hubble and the James Webb Space Telescope to better constrain the motions of the stars within it.
The findings were published in Monthly Notices of the Royal Astronomical Society.
