Amidst the star ball floating in the Milky Way, there was a monster lurking.
About 6,000 light years away, the globular cluster known as Messier 4 appears to be clustered around a black hole that is 800 times the mass of our Sun.
It’s not a featherweight, but it’s far from a giant either. In fact, the body is in the rarely seen middle mass range, between the smallest black hole and super oversized chokers.
To date, our only discoveries about these black holes have been neither straightforward nor conclusive, and this one is no exception.
Still, it’s one of the best candidates so far, and close enough that follow-up studies can be done with relative ease. This could help us finally find one of these elusive objects definitively, and solve one of the most perplexing mysteries of black holes.
“Science is rarely about discovering something new in one moment.” said astronomer Timo Brosti European Space Agency. “It’s about being more certain of the results step by step, and that might be one step toward confirming that intermediate-mass black holes do exist.”
We’ve identified too many black holes in the universe, and something very strange is 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 are located at the cores of galaxies and bind millions to billions of suns.
Between those two mass ranges… too much, not much, really. This raises a big conundrum, which is basically, why not? Are there no intermediate-mass black holes? Or are they there, and we can’t find them for some reason?
We know how stellar-mass black holes form – the fundamental collapse of massive stars, and the fusion between these objects. But we’re not too sure about that Supermassive black hole formation. Do they grow from successive mergers of smaller black holes, or do they absorb matter and swell?
Intermediate-mass black holes may be a clue, suggesting they can start small and grow to become massive over time. This would certainly make a lot of sense, but the lack of these ideas is a very effective barrier to this idea.
One possible site Where this black hole might be hiding is in the heart of a globular cluster. It is an unusually dense and extremely round cluster of between 100,000 and a 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 a high concentration of mass at their center consistent with the mass range of an intermediate-mass black hole.
Messier 4 is the closest globular cluster to Earth. Led by astronomer Eduardo Vitral of the Space Telescope Science Institute, the research team used two powerful space telescopes, Hubble and Gaia, to get a closer look at the stars inside. They tracked the motions of about 6,000 stars in the cluster, to see if they could correlate these motions with orbits around the small, dense cluster.
We usually can’t see a black hole if matter isn’t actively accreting it, but the orbit would provide a pretty reliable guide. Their calculations revealed something, with a mass of about 800 solar masses. Although what this object is may not be clear.
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“Using the latest Gaia and Hubble data, it is impossible to distinguish a dark remnant star cluster from a single, larger point-like source,” Vitral said. “So one possible theory is that instead of being many separate small dark objects, this dark mass could become a medium-sized black hole.”
To try to narrow it down, the team performed star modeling and displacement to see how that changed the shape of the cluster. Removing a fast-moving star spreads its mass over greater distances, as you might see from a swarm of small black holes and neutron stars. Further modeling shows that the clusters do not spread over a large enough area to become a swarm.
On top of that, a swarm of black holes would be so close together that they would essentially create chaos. The gravitational interaction will cause the star to fly out of the cluster, smear it anarchism across the sky. We may, in fact, have seen the effect in a file The star cluster is called Palomar 5.
“We firmly believe that we have a very small area with a lot of concentrated masses. It is about three times smaller than the denser dark masses we have found before in other globular clusters.” Vitral said.
“While we cannot really say that it is the center of gravity, we can show that it is very small. Too small for us to be able to describe it otherwise as a single black hole. Alternatively, there may be a star mechanism that we don’t know about, at least in current physics.”
So barring new physics or invisible stars, an intermediate-mass black hole seems the most likely explanation at the moment. But the existence of a number of small black holes is still a realistic explanation. The researchers recommend further observing the cluster using the 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.
2023-05-25 09:06:54
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