Astronomers discover supermassive black hole population in Palomar 5 star cluster

In a stunning discovery, astronomers have detected oversized black hole population in one of the "fluffiest" clusters in the halo of our Galaxy. An international team of astrophysicists led by the University of Barcelona  conducted a research on unique globular star cluster ‘Palomar 5’ located at approximately 80,000 light-years and found that it houses as many as hundreds of massive black holes.

The stellar cluster was first discovered by Walter Baade in 1950 and is situated in the Serpens constellation. It is one of the  “spongiest” and most unique globular cluster that was created over ten billion years ago in the earliest phases of galaxy formation in the Milky Way. In a study published Tuesday, July 6, scientists explained that Palomar 5 had initially formed with a lower black hole fraction, however the percentage of black holes drastically increased as the stars escaped from the centre of the cluster.

Professor Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the paper published in Nature Astronomy 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 percent of the total cluster mass is made up of black holes.” Furthermore, he added, “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.” 

[Palomar 5 star cluster. Credit: twitter/@RadioAllen]

“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 cannot be sure,” said the lead author of the paper Professor Mark Gieles.

He continued, “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.”

'Cluster will be entirely black holes' scientists predict 

As scientists studied the orbits and the evolution of each star from the formation of the cluster, they found that the black holes dynamically puffed up the cluster in gravitational slingshot interactions with stars. As a result, more percentage of stars started to escape, forming a stream-like structure. “ Roughly a billion years from now, the cluster will consist entirely of black holes,” the scientists noted. Palomar 5 cluster now has the brightest and longest tails of any cluster in the Milky Way, and is significantly a black hole-dominated clusters.

It may not be unique, says co-author Dr. Denis Erkal at the University of Surrey. Such puffed up, black hole clusters may have already disintegrated in the Milky Way to form similar such recently discovered thin stellar streams, he added. “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,” said Dr. Fabio Antonini from Cardiff University, a co-author of the paper.