Palomar 5 Contains More Than 100 Stellar-Mass Black Holes: Study | Astronomy
Palomar 5 is one of the sparsest star clusters in the halo of our Milky Way Galaxy and is best known for its spectacular tidal tails, spanning over 20 degrees across the sky.
Palomar 5 is a globular cluster located some 80,000 light-years away in the constellation of Serpens.
Discovered in 1950 by the German astronomer Walter Baade, it is approximately 11.5 billion years old.
Palomar 5 has an unusually large half-light radius of65 light-years and, combined with its relatively low mass of 10,000 solar masses, its average density is among the lowest of all globular clusters in the Milky Way.
It is one of the few known star clusters with extended tidal tails associated with it, spanning over 20 degrees on the sky, making it a Rosetta stone for understanding formation of tidal tails and streams.
We do not know how these streams/tails form, but one idea is that they are disrupted star clusters, said Professor Mark Gieles, an astronomer in the Institute of Cosmos Sciences at the University of Barcelona and ICREA.
However, none of the recently discovered streams/tails have a star cluster associated with them, hence we can not be sure.
So, to understand how they formed, we need to study one with a stellar system associated with it.
In the new study, Professor Gieles and colleagues simulated the orbits and the evolution of each star from the formation of Palomar 5 until the final dissolution.
They varied the initial properties of the cluster until a good match with observations of the tails and the cluster was found.
They found 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 number of black holes — more than 100 such compact objects — is roughly three times bigger 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, Professor Gieles said.
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.
The black holes dynamically puffed up Palomar 5 in gravitational slingshot interactions with stars, which led to even more escaping stars and the formation of the tails.
The astronomers predict that a billion years from now, the cluster will dissolve as a 100% black hole cluster.
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, said Dr. Denis Erkal, an astronomer in the Department of Physics at the University of Surrey.
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.
It is believed that a large fraction of binary black hole mergers form in star clusters, said Dr. Fabio Antonini, an astronomer in the Gravity Exploration Institute at Cardiff University.
A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we can not 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.
The study was published in the journal Nature Astronomy.
M. Gieles et al. A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5. Nat Astron, published online July 5, 2021; doi: 10.1038/s41550-021-01392-2