At the center of a ball 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 clustering around a black hole 800 times the mass of our Sun.
This isn’t a featherweight, but it’s far from gigantic, either. In fact, the body is in the rarely seen middle mass range, between the smallest black holes and super bulky chonkers.
To date, our only detections of these black holes have been indirect and inconclusive, and this discovery is no exception.
However, it is one of the best candidates so far, and close enough that a follow-up study can be done relatively easily. This may help us finally find one of these elusive objects definitively, and solve one of the most puzzling mysteries of black holes.
“Science is rarely about discovering something new in one moment.” says astronomer Timo Brosti European Space Agency. “It’s about becoming more certain of a result step by step, and that might be one step toward making sure that intermediate-mass black holes exist.”
We’ve identified far too many black holes in the universe, and there’s something very strange going on their mass distribution. There are two distinct populations: stellar-mass black holes, which are about 100 times the mass of the Sun; and supermassive black holes, which lie at the cores of galaxies and girdle millions to billions of suns.
Between those two mass ranges…too many not much at all, really. This poses a huge puzzle, which is basically, why not? Are there no intermediate mass black holes? Or are they just there, and we can’t spot them for some reason?
We know how stellar-mass black holes form – the fundamental collapse of massive stars, and the 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 may be a clue, suggesting that they can start small and grow massive over time. It would certainly make a lot of sense, but the paucity of these ideas is a very effective impediment to this idea.
One possible site Where these black holes may be lurking in the hearts of globular clusters. These are incredibly dense, remarkably spherical clusters of about 100,000 to a million or so stars, which formed mostly at the same time from the same gas cloud. Previous studies focusing on globular clusters have found high concentrations of mass in 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, the Hubble and Gaia, to get a closer look at the stars within. They tracked the motions of about 6,000 stars in the cluster, to see if they could correlate those motions with orbits around the small, dense cluster.
We can’t normally see black holes if matter isn’t actively accreting, but those orbits would be a pretty reliable guide. Their calculations revealed something, with a mass of about 800 solar masses. Although what this thing might be is not clear.
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“Using the latest Gaia and Hubble data, it was not possible to distinguish a dark cluster of remnant stars from a single, larger point-like source,” Vitral says. “So one possible theory is that instead of it being a lot of separate small dark objects, this dark mass could be an intermediate-sized black hole.”
To try to narrow it down, the team performed star modeling and removal to see how that changes the shape of the cluster. Removing a particularly fast-moving star spreads the mass over a greater distance, as you might see from a swarm of small black holes and neutron stars. Further modeling showed that the cluster does not spread across an area large enough to be such a swarm.
Plus, a swarm of black holes would be so close together that they would basically cause chaos. Gravitational interactions will cause stars to fly out of the cluster, smear it anarchism across the sky. We may, in fact, have already seen the effects of this in a file The star cluster called 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 denser dark mass we’ve found before in other globular clusters,” Vitral says.
“While we can’t absolutely assert that it is a central point of gravity, we can show that it is very small. It is too small for us to be able to explain other than it being a single black hole. Alternatively, there may be a stellar mechanism that we know nothing about, At least in current physics.”
So, barring new physics or invisible stars, an intermediate-mass black hole seems to be the most likely explanation at the moment. However, the existence of a number of small black holes is still a realistic explanation. The researchers recommend further observations of the cluster using Hubble and the James Webb Space Telescope to better constrain the motions of the stars within it.
The results are published in Monthly Notices of the Royal Astronomical Society.
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