Astronomers have found an oversized black hole population in a star cluster Palomar 5 – roughly three times the number of black holes than expected.
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 5 times more extended than a typical globular cluster and in the final stages of dissolution.
According to astronomers more than 20% of the total cluster mass is made up of black holes with each of them having a mass of about 20 times the mass of the Sun.
According to estimates, each of these black holes were 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.
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. The black holes dynamically puffed up the cluster in gravitational slingshot interactions with stars, which led to even more escaping stars and the formation of the stream. Just before it completely dissolves – roughly a billion years from now – the cluster will consist entirely of black holes.
Researchers point out that in this paper they have shown that the presence of a large black hole population may have been common in all the clusters that formed the streams. This is important for our understanding of globular cluster formation, the initial masses of stars and the evolution of massive stars. This work also has important implications for gravitational waves.