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Palomar 5 is an unusual star cluster. A paper published in Nature Astronomy today by an international team of scientists led by the University of Barcelona shows that Palomar 5's distinctive features are likely due to a large black hole population of over 100 in the cluster.
"The number and mass of black holes in the cluster is three times greater than what we expected based on the number of stars. This means that black holes make up more than 20% of total cluster mass. Each of them has a mass of approximately 20 times the Sun's. They formed in supernova explosions when the cluster was very young," said Prof Mark Gieles from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), who is the lead author of the paper.
Tidal streams are stars that have been ejected from star clusters or dwarf galaxies. Nearly thirty thin streams were discovered in the Milky Way's halo over the past few years. We don't know how these streams formed, but we do know that they may be disrupted star clusters. We are not able to be certain because none of the streams we have recently discovered has a star cluster associated. To understand how these streams form, we must study the one that has a stellar system. Palomar 5 is the unique case. It is a Rosetta Stone to understanding stream formation, which is why we studied it in depth," Gieles explains.
They simulate the orbits of each star and their evolution from formation to dissolution. The initial properties of the cluster were varied until they were in line with observations of the stream. The team discovered that Palomar 5 was formed with a lower percentage of black holes. However, stars were more efficient than black holes and the black hole fraction slowly increased. The black holes dynamically puff up the cluster through gravitational slingshot interactions and stars, which resulted in more stars escaping and the formation the stream. It will be made entirely of black holes just before it dissolves. This is approximately a billion years away. "This research has helped us to understand that the Palomar 5 cluster, which has the longest and brightest tails in the Milky Way, is not the only one. We believe that other similarly puffy, black-hole-dominated clusters may have disintegrated in Milky Way tides to create the newly discovered thin stellar streams," Dr. Denis Erkal, University of Surrey, co-author.
Gieles states, "We have shown the existence of a large population of black holes may have been common among all the clusters which formed the streams." This information is crucial for understanding globular cluster formation, star formation and the evolution of large stars. This research also has important implications regarding gravitational waves. It is believed that binary black hole mergers are common in star clusters. The number of black holes in clusters is a big mystery in this scenario. It is difficult to determine observationally as we cannot see black holes. We can use our method to determine how many BHs are present in a star cluster, by looking at what stars they eject. "," says Dr. Fabio Antonini, Cardiff University co-author.
Walter Baade discovered Palomar 5, a globular cluster, in 1950. It orbits around the Milky Way at approximately 80,000 light years. It can be found in the Serpens constellation. Like most globular clusters it is more than 10 billion years old. This means that it formed during the early stages of galaxy formation. It is approximately 10 times smaller and five times longer than the typical globular cluster, and it is in the final stages to dissolution.
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More information: A supra massive population of stellar mass black holes within the globular cluster Palomar 5. Nature Astronomy (2021). Information from Nature Astronomy: A supra-massive population of stellar-mass dark holes in the globular Cluster Palomar 5 (2021). DOI: 10.1038/s41550021-01392-2