At the heart of a globe of stars drifting through the Milky Way lies a beast.
Located some 6,000 light-years away, a globular cluster known as Messier 4 appears to cluster around a black hole some 800 times the mass of our Sun.
It’s not a featherweight, but it’s far from being a colossus either. In fact, the object falls into a rarely seen intermediate mass range, between the smallest black holes and supermassive chonkers.
So far, our only detections of these intermediaries black holes have been mostly indirect and inconclusive, and this one is no exception.
It is, however, one of the best candidates to date, and close enough that a follow-up study can be conducted relatively easily. This could help us finally conclusively find one of these elusive objects and solve one of the most baffling black hole mysteries.
“Science is rarely about discovering something new in a single instant,” says astronomer Timo Prusti of the European Space Agency. “It’s about becoming more certain of a step-by-step conclusion, and it could be a step towards certainty that intermediate-mass black holes exist.”
We have identified quite a 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 sit at the hearts of galaxies and point millions or even billions of suns away.
Between these two mass ranges, there is… a lot of not much at all, really. This is a big conundrum, which is basically, why the hell not? Aren’t there just no intermediate-mass black holes? Or are they out there, and we just can’t detect them for some reason?
We know how stellar-mass black holes form – the collapse of the cores of massive stars and the mergers between these objects. But we’re not so sure formation of supermassive black holes. Do they come from successive mergers of smaller black holes or do they suck up matter and grow?
Intermediate-mass black holes would be one clue, suggesting they may start small and grow larger over time. It would certainly make a lot of sense, but their scarcity is a pretty effective barrier to this idea.
A possible location where these black holes might be hiding is in the heart of globular clusters. They are incredibly dense and remarkably spherical clusters of about 100,000 to about 1 million stars, which mostly formed at the same time from the same cloud of gas. Previous studies have focused on globular clusters have found high concentrations of mass in their centers conform to the mass ranges 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 get a close look at the stars inside. They tracked the movements of about 6,000 stars in the cluster, to see if they could relate these movements to orbits around a small, dense mass.
We usually can’t see black holes if they’re not actively accumulating matter, but these orbits would be a pretty reliable clue. And their calculations revealed something, with a mass of about 800 solar masses. Although what that something might be is unclear.
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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 single larger point source,” Stained glass says. “So one of the possible theories is that rather than being a lot of separate small dark objects, this dark mass could be a medium-sized black hole.”
To try to narrow it down, the team did some modeling, removing the stars to see how that changes the shape of the mass. Removing a particularly fast star spreads the mass over a greater distance, as you can see in a swarm of small black holes and neutron stars. Further modeling showed that the mass is not distributed over a large enough region of space to constitute such a swarm.
Moreover, a swarm of black holes would be so close together that they would essentially create a mess. Gravitational interactions would send stars flying out of the cluster, spread it chaotically across the sky. In fact, we may have already seen the effects of this in a star cluster named Palomar 5.
“We are 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 we have found before in other globular clusters,” Stained glass says.
“Although we cannot completely state that it is a central point of gravity, we can show that it is very small. It is too small for us to explain otherwise that it is a central point of gravity. “a single black hole. Alternatively, there could be a stellar mechanism that we simply don’t know about, at least in current physics.”
So, barring new physics or invisible stars, an intermediate-mass black hole seems like the most likely explanation for now. Nevertheless, a population of small 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 inside.
The findings were published in the Royal Astronomical Society Monthly Notices.