Discovered A Population of Large Black Holes in the Star Cluster Palomar 5 (Cosmology)

Palomar 5 is a unique star cluster. This is due, in the first place, to the fact that it is one of the “spongiest” in the Milky Way halo, with an average distance of a few light-years between the stars, comparable to the distance from the Sun to the brightest star. next. Second, because it is associated with a specular stellar current that covers more than 20 degrees in the sky. In an article published in Nature Astronomy , an international team of astronomers and astrophysicists led by the University of Barcelona shows that the two distinctive features of Palomar 5 are probably the result of a population of more than one hundred black holes in the center of the cluster.

“The number of black holes is about three times greater than would be expected from the number of stars in the cluster, which means that more than 20% of the total mass of the cluster is made up of black holes,” explains Mark Gieles . professor at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the work. “Each has a mass twenty times the mass of the Sun,” says the expert, “and they formed in supernova explosions at the end of the life of massive stars, when the cluster was still very young.”

Tidal currents are associations of stars that were ejected from star clusters or dwarf galaxies. In recent years, about thirty narrow stellar currents have been discovered in the halo of the Milky Way. “We don’t know how these currents form, but one idea is that they are star clusters that have suffered some disturbance,” explains Gieles. However, none of the recently discovered currents have an associated star cluster, so researchers cannot be sure of this theory. To understand how these currents formed, one must study one with an associated stellar system: “Palomar 5 is the only case, and this makes it a kind of Rosetta Stone, which will allow us to understand the formation of stellar currents. · Lars », points out Gieles. “That’s why we studied it in detail,” he explains.

CCUB researcher Mark Gieles. 
Image: ICCUB

In this study, the authors simulated the orbits and evolution of each star from cluster formation to final dissolution. Their initial properties varied until they found that the observations of the current and the cluster matched. The astronomer team believes that Palomar 5 was formed from a smaller black hole fraction, but the stars were able to escape more efficiently than the black holes, so the black hole fraction increased gradually.

The black holes dynamically inflated the cluster through gravitational assistance interactions with the stars, which caused more stars to escape and the current to form. Just before it dissolves completely — about a billion years from now — the cluster will be completely made up of black holes. “This work has helped us to understand that, although the Palomar 5 cluster has the brightest and longest tails than any other cluster in the Milky Way, it is not unique. Instead, we believe that many similarly dominated black hole clusters have already disintegrated in the tides of the Milky Way to form the first newly discovered stellar currents, “said Denis Erkal , co-author of the study. , researcher at the University of Surrey (UK).

Gieles notes that the study shows “that the presence of a large population of black holes may have been common in all groups that formed the currents.” This is important for understanding the formation of globular clusters, initial star masses, and the evolution of massive stars. This work also has important implications for gravitational waves. “A large portion of binary black hole fusions are believed to form in star clusters. A big unknown in this scenario is the number of black holes in the clusters, which is difficult to delimit observationally because we can not observe the black holes, “says Fabio Antonini, Professor at Cardiff University (Wales, UK), and also co-author of the paper. “Our method provides a way to know how many black holes there are in a star cluster by looking at the stars that are ejected from it,” he said.

Palomar 5 is a globular cluster discovered in 1950 by Walter Baade. It is located in the constellation of the Serpent, at a distance of about 65,000 light-years, and is one of approximately 150 globular clusters orbiting the Milky Way. It is over ten billion years old, like most other globular clusters, meaning it formed in the early stages of galaxy formation. It is approximately ten times less massive and five times more extensive than a typical globular cluster and in the final stages of dissolution.

Simulation showing the formation of tidal currents in the Palomar 5 star cluster and the distribution of black holes. In yellow you see the stars and in black the black holes. More than 20% of the mass of Palomar 5 is made up of black holes.

Featured image: Map of the Milky Way plan obtained from data in the Gaia catalog (eDR3). At the top is a region where the star cluster of Palomar 5 and its tidal tails are observed (data obtained thanks to the DESI Legacy Imaging Survey, DECaLS). Image: E. Balbinot, Gaia, DECaLS-DESI


Reference article :

M. Gieles et al. “A supra-massive population of Stella-massblack holes in the globular cluster Palomar 5” . Nature Astronomy , July 5, 2021. DOI: 10.1038 / s41550-021-01392-2


Provided by University of Barcelona

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