Palomar 5 is a one-of-a-kind nebula. This is due to the fact that it is one of the “fluffiest” clusters in our Galaxy’s halo, with an average gap between stars of a few light-years, about equivalent to the distance between the Sun and the nearest star. Second, it is connected with a specular stellar stream that covers greater than 20 degrees across the sky.
An international team of astronomers and astrophysicists led by the University of Barcelona show in a paper published today in Nature Astronomy that both of Palomar 5’s distinguishing features are likely the result of an oversized black hole population of more than 100 black holes in the cluster’s center.
“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,” says Prof Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the paper.
Tidal streams are streams of stars released from star clusters or dwarf galaxies that have been disrupted. Nearly thirty thin streams have been identified in the Milky Way halo in the last several years.
“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 have a star cluster associated with them, hence we can not be sure. So, to understand how these streams 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.
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 can not see black holes. Our method gives us a way to learn how many BHs there are in a star cluster by looking at the stars they eject.
Dr. Fabio Antonini
From the creation of the cluster to its final dissolution, the authors model the orbits and history of each star. They tweaked the cluster’s initial parameters until they found a good match between the stream’s observations and the cluster.
Palomar 5 was discovered to have a smaller black hole fraction than other galaxies, but stars escaped more efficiently than black holes, therefore the black hole fraction progressively grew. In gravitational slingshot interactions with stars, the black holes dynamically inflated up the cluster, resulting in, even more, escape stars and the creation of the stream. The cluster will be fully made up of black holes just before it completely dissipates in around a billion years.
“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,” says co-author Dr. Denis Erkal at the University of Surrey.
Gieles points out that in this paper “we 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 crucial for our knowledge of globular cluster formation, stellar starting masses, and massive star evolution. This research has consequences for gravitational waves as well.
“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 can not see black holes. Our method gives us a way to learn how many BHs there are in a star cluster by looking at the stars they eject.” says Dr. Fabio Antonini from Cardiff University, a co-author of the paper.
Walter Baade discovered Palomar 5 as a globular cluster in 1950. It is one of the about 150 globular clusters that revolve around the Milky Way, and it is located in the Serpens constellation at a distance of around 80,000 light-years.
It, like most other globular clusters, is older than 10 billion years, indicating that it formed during the early stages of galaxy formation. It’s about 10 times less massive and 5 times longer than a typical globular cluster, and it’s nearing the conclusion of its life cycle.
