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A fluffy cluster of stars that spreads across the sky may be hiding a secret at its heart: a swarm of more than 100 stellar-mass black holes. The star cluster in question is called Palomar 5. It's a stream of stars that spans 30 light-years and lies about 000 light-years away. Globular clusters like these are often considered "fossils" of the early universe. They are very dense and spherical, typically containing about 80 to 000 million very old stars; some, like NGC 100, are almost as old as the universe itself.
In any globular cluster, all its stars formed at the same time from a single cloud of gas. There are over 150 globular clusters known in the Milky Way; these objects are excellent tools for studying, for example, the history of the Universe or the dark matter content of the galaxies around which they orbit. But there is another type of star group that is attracting more attention: tidal streams, long rivers of stars that stretch across the sky. They have previously been difficult to identify, but thanks to data from the Gaia space observatory, which has mapped the Milky Way with high precision in three dimensions, more of these streams have been discovered.
“We don’t know how these streams form, but one idea is that they are broken star clusters,” explained astrophysicist Marc Giles of the University of Barcelona in Spain in 2021, when the researchers first announced the discovery. “However, none of the newly discovered streams have an associated star cluster, so we can’t be sure. So to understand how these streams formed, we need to study one with an associated star system. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation, and that’s why we studied it in detail.”
Palomar 5 appears unique in that it has both a very broad, sparse distribution of stars and a long tidal stream spanning more than 20 degrees of sky, so Giles and his team focused on it. The team used detailed N-body simulations to recreate the orbits and evolution of each star in the cluster to see how they might have ended up 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 scatter, the scientists included black holes in some of their simulations.
Their results showed that a population of stellar-mass black holes inside Palomar 5 could have led to the configuration we see today. Orbital interactions would have ejected stars from the cluster into the tidal stream, but only with a much larger number of black holes than predicted. Stars escaping the cluster more efficiently and more easily than black holes would have altered the proportion of black holes, greatly increasing it.
"The number of black holes is about three times the expected number of stars in the cluster, meaning that more than 20 percent of the total mass of the cluster is made up of black holes," Giles said. "Each one has a mass about 20 times that of the Sun, and they were formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young."
In about a billion years, the team's simulations show that the cluster will completely dissolve. Shortly before that happens, what remains of the cluster will consist entirely of black holes orbiting the galactic center. This suggests that Palomar 5 is not unique, as it will completely dissolve into the stellar stream, like the others we have discovered.
It also suggests that other globular clusters are likely to share the same fate, eventually. And it's confirmation that globular clusters could be a great place to look for black holes that will eventually collide, as well as an elusive class of intermediate-mass black holes, between the lightweight stellar-mass ones and the supermassive ones.
"A large fraction of binary black hole mergers are thought to form in star clusters," said astrophysicist Fabio Antonini of Cardiff University in the UK. "The big unknown in this scenario is the number of black holes in the clusters, which is difficult to constrain with observations because we can't see the black holes. Our method gives us a way to know how many black holes there are in a star cluster by looking at the stars they eject." The study is published in Nature Astronomy.
