Palomar 5 is a unique star cluster. In a paper published today in Nature Astronomy, an international team of astrophysicists led by the University of Barcelona show that distinguishing features of Palomar 5 are likely the result of an oversized black hole population of more than 100 of them in the centre of the cluster.
Prof Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona and lead author of the papers, said that the number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20% of the total cluster mass is made up of black holes. They each have 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.
Tidal streams are streams of stars that were ejected from disrupting star clusters or dwarf galaxies. In the last few years, nearly thirty thin streams have been discovered in the Milky Way halo. They do not know how these streams form, but one idea is that they are disrupted star clusters. The discovered streams have a star cluster associated with them, hence we can not be sure.The authors simulate the orbits and the evolution of each star from the formation of the cluster until the final dissolution. They varied the initial properties of the cluster until a good match with observations of the stream and the cluster was found. The team finds that Palomar 5 formed with a lower black hole fraction, but stars escaped more efficiently than black holes, such that the black hole fraction gradually increased.
Palomar 5 is a globular cluster discovered in 1950 by Walter Baade. It is in the Serpens constellation at a distance of about 80,000 light-years, and it is one of the roughly 150 globular clusters that orbit around the Milky Way. It is older than 10 billion years, like most other globular clusters, meaning that it formed in the earliest phases of galaxy formation. It is about 10 times less massive and five times more extended than a typical globular cluster and in the final stages of dissolution.