Astronomers discover an oversized black hole population in the star cluster Palomar 5

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 from Earth, and it is one of the roughly 150 globular clusters that orbit around the Milky Way. Like most other globular clusters it is older than 10 billion years, dating from 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. 

In a paper published today in Nature Astronomy, the international team, including members from the University of Surrey, shows that distinguishing features of Palomar 5 are likely the result of an oversized black hole population of more than 100 black holes. 

Professor Mark Gieles, lead author of the paper from the Institute of Cosmos Sciences at the University of Barcelona, said:

“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 per cent of the total cluster mass is made up of black holes. The black holes each have a mass of about 20 times that 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.” 

Further, around thirty thin tidal streams have been discovered in the Milky Way halo in recent years. Tidal streams are a collection of stars ejected from disrupting star clusters or dwarf galaxies.

“We do not know how these streams form, but one idea is that they are disrupted star clusters. However, none of the recently discovered streams has a star cluster associated with them; hence we cannot be sure. So, to understand how these streams are formed, we need to study one with a stellar system associated with it. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation, and that is why we studied it in detail,” explains Gieles.

The authors simulated 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 found that Palomar 5 formed with a lower black hole fraction, but stars escaped more efficiently than black holes, so much so that the number of black holes 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. 

Dr Denis Erkal, co-author of the paper from the University of Surrey, said: “This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique. Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams.” 

Dr Fabio Antonini, co-author of the paper from Cardiff University, said: “It is believed that a large fraction of binary black hole mergers form in star clusters. A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we cannot see black holes. Our method gives us a way to learn how many black holes there are in a star cluster by looking at the stars they eject.”

Read the full report here.