In the center of a mass of stars drifting 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.
It’s not a featherweight, but it’s also far from a colossus. In fact, the object falls into a rarely seen intermediate mass band, between smaller black holes and supermassive chonkers.
So far, our only detections of these intermediate black holes have been indirect and inconclusive, and this one is no exception.
It is, however, one of the best candidates so far, and close enough that a follow-up study could be carried out relatively easily. It could help us finally find one of these elusive objects and solve one of the most baffling 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 being more sure of a step-by-step conclusion, and this could be a step towards being 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 about 100 times the mass of the Sun; and supermassive black holes, which lie at the heart of galaxies and span millions to billions of suns.
In between those two ranges of mass is… very little, actually. This constitutes a huge conundrum, which is basically, why the hell not? Are there simply 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 – the core collapse of massive stars and mergers between these objects. But we’re 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 increase in size?
Intermediate-mass black holes would be a clue, suggesting they might start out small and grow larger over time. It would certainly make a lot of sense, but their scarcity is a pretty effective deterrent to this idea.
One possible place where these black holes could be lurking is in the heart of globular clusters. These are incredibly dense and remarkably spherical clusters of anywhere from 100,000 to 1 million stars, which formed at the same time from the same cloud of gas. 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 closely observe the stars within. They tracked the movements of about 6,000 stars in the cluster, to see if they could link those movements to orbits around a small, dense mass.
Normally, we can’t see black holes if they aren’t actively accumulating matter, but these orbits would provide a pretty reliable clue. And his calculations revealed something, with a mass of about 800 solar masses. Though what that something might be is unclear.
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“Using the most recent data from Gaia and Hubble, it was not possible to distinguish between a dark population of stellar remnants and a single larger point source,” says Vitral. “So one of the possible theories is that instead of being a bunch of separate small dark objects, this dark mass could be a medium-sized black hole.”
To try to narrow it down, the team performed modeling, removing stars to see how this changes the shape of the mass. Removing a star particularly quickly spreads the mass over a greater distance, as you can see in a swarm of smaller black holes and neutron stars. Further modeling showed that the mass is not spread over a region of space large enough to be a swarm.
Also, a swarm of black holes would be so close together that it would basically create a mess. Gravitational interactions would send stars flying out of the cluster, staining it chaotically across the sky. We may, in fact, already have seen the effects of this in a star cluster called Palomar 5.
“We have good confidence 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 center point of gravity, we can show that it’s very small. It’s too small for us to explain beyond being a single black hole. Alternatively, there could be a star mechanism that we simply don’t know about, at least inside. of current physics.”
So, barring new physics or unseen 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 stars within it.
The findings were published in Monthly Notices of the Royal Astronomical Society.