A spectacular cluster of stars spilling across the sky may have a secret hidden in its “heart”: a cluster of more than 100 stellar-mass black holes.
The star cluster in question is called Palomar 5. It is a star stream that spans 30,000 light-years and is located about 80,000 light-years away. Such globular clusters are often considered “fossils” of the early Universe. They are very dense and spherical, typically containing between 100,000 and 1 million very old stars; some, like NGC 6397, are almost as old as the Universe itself.
In any globular cluster, all the stars formed at the same time, from the same cloud of gas. The Milky Way has about 150 known globular clusters; these objects are excellent tools for studying, for example, the history of the Universe or the dark matter content of the galaxies they orbit.
But there’s another type of star cluster that’s gaining more and more attention—tidal streams, long rivers of stars that stretch across the sky. Previously, they were difficult to identify, but with the Gaia space observatory, which is working to map the Milky Way with high precision in three dimensions, more of these streams have been brought to light.
“We don’t know how these streams form, but one idea is that they are disrupted star clusters,” astrophysicist Mark Gieles of the University of Barcelona, Spain, explained in 2021 when researchers first announced the discovery.
The star cluster in question is called Palomar 5
“However, none of the recently discovered streams have an associated star cluster, so we cannot be sure. So to understand how these streams formed, we need to study one with a star system associated with it. Palomar 5 is the only case, which makes it a Rosetta stone for understanding flow formation, and that’s why we studied it in detail.”
Palomar 5 appears unique in that it has both a very broad and loose distribution of stars and a long tidal stream that spans more than 20 degrees across the sky, so Gieles and his team focused on to him, he writes ScienceAlert.
The team used detailed N-body simulations to recreate the orbits and evolutions of each star to see how they might have gotten to where they are today.
Because recent evidence suggests that populations of black holes may exist in the central regions of globular clusters, and because gravitational interactions with black holes are known to cause stars to drift apart, the researchers included black holes in some of their simulations.
Their results showed that a population of stellar-mass black holes in Palomar 5 could have led to the configuration we see today. Orbital interactions would have pulled the stars out of the cluster and thrown them into the tidal stream, but only with a significantly larger number of black holes than predicted.
Palomar 5 will dissolve completely
Stars escaping the cluster more efficiently and easily than black holes would have altered the proportion of black holes, increasing it quite a bit.
“The number of black holes is about three times higher than expected based on the number of stars in the cluster, and means that more than 20% of the cluster’s total mass consists of black holes,” said Gieles. “Each of these has a mass of about 20 times the mass of the Sun, and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young.”
The team’s simulations showed that in about a billion years, the swarm will completely dissolve. Even before this happens, what will be left of it will consist entirely of black holes, orbiting the galactic center. This suggests that Palomar 5 is not unique after all – it will dissolve completely into a stellar stream, just like others we have discovered.
It also suggests that other globular clusters will likely suffer the same fate eventually. It also confirms that globular clusters can be excellent places to search for black holes that will eventually collide, as well as the elusive class of intermediate-mass black holes, between low-stellar-mass and high-mass supermassive black holes.
The research was published in Nature Astronomy.
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Source: www.descopera.ro
