In the heart of a globe of stars moving through the Milky Way lurks a beast.
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 not a colossus either. In fact, the object falls into an intermediate mass range that is rarely seen, between the smallest black holes and supermassive chonkers.
So far, our only detections of these intermediate black holes have been mostly indirect and inconclusive, and this one is no exception.
However, it is one of the best candidates so far, and close enough that follow-up study can be done relatively easily. This could help us finally conclusively find one of these elusive objects and solve one of the most perplexing mysteries of black holes.
“Science is rarely about discovering something new in a single moment,” says astronomer Timo Prusti of the European Space Agency. “It’s about reaching a more certain conclusion step by step, and this could be a step to be sure that intermediate-mass black holes exist.”
We have identified a large number of black holes in the Universe, and there is something very strange about their mass distribution. There are two distinct populations: stellar-mass black holes, up to around 100 times the mass of the Sun; and supermassive black holes, which sit at the heart of galaxies and clock in at millions to billions of suns.
In the middle of these two ranges of mass is…a lot of nothing, actually. This is a big conundrum, which is basically, why the hell not? Are there just no intermediate-mass black holes out there? Or are they out there and we just can’t detect them for some reason?
We know how stellar-mass black holes form: core collapse of massive stars and mergers between these objects. But we are not so sure about the formation of supermassive black holes. Do they grow from successive mergers of smaller black holes, or do they suck in material and grow larger?
Intermediate-mass black holes would be one clue, suggesting they may start small and grow larger over time. Sure it would make a lot of sense, but the scarcity of them is a pretty effective roadblock to this idea.
One possible location where these black holes could lurk is at the heart of globular clusters. These are incredibly dense and remarkably spherical clusters of around 100,000 to 1 million stars, most of which formed at the same time from the same gas cloud. Previous studies focused 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 close look at the stars within. They tracked the motions of around 6,000 stars in the cluster, to see if they could link those motions to orbits around a small, dense mass.
We usually can’t see black holes if they’re not actively accreting matter, but those orbits would be a fairly reliable clue. And his calculations revealed something, with a mass of about 800 solar masses. Although it is not clear what that something could be.
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“Using the latest data from Gaia and Hubble, it was not possible to distinguish between a dark population of stellar remnants and a larger single point source,” says Vitral. “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 reduce it, the team performed modeling, removing stars to see how that alters the shape of the mass. By removing a particularly fast-moving star, the mass is spread out over a greater distance, as might be seen in a swarm of smaller black holes and neutron stars. Later modeling showed that the mass does not extend across a large enough region of space to be a swarm.
Also, a swarm of black holes would be so close to each other that they would essentially create a mess. Gravitational interactions would send stars flying out of the cluster, spreading it chaotically Through the sky In fact, we may have already seen the effects of this in a star cluster called Palomar 5.
“We are very confident that we have a very small region with a lot of concentrated mass. It is about three times smaller than the densest dark mass that we have found before in other globular clusters,” says Vitral.
“While we can’t fully state that it’s a central point of gravity, we can show that it’s very small. It’s too small for us to explain other than it being a single black hole. Alternatively, there could be a stellar mechanism that we just don’t know about, at least within current physics”.
So, barring new physics or invisible stars, an intermediate-mass black hole seems the most likely explanation for now. However, a population of smaller black holes is still a realistic 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 have been published in the Monthly Notices of the Royal Astronomical Society.
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