Astronomers have discovered a swarm of over 100 stellar-mass black holes moving through the Milky Way.
This discovery was made in the heart of Palomar 5, a globular cluster of stars located around 80,000 light-years away. Globular clusters are dense, spherical collections of stars, often considered 'fossils' of the early Universe. Palomar 5 is unique in that it has a very wide, loose distribution of stars and a long tidal stream, making it a key subject for understanding stream formation.
The team of researchers used detailed N-body simulations to recreate the orbits and evolutions of each star in the cluster. They included black holes in their simulations, as recent evidence suggests that populations of black holes could exist in the central regions of globular clusters. The results showed that a population of stellar-mass black holes within Palomar 5 could have resulted in the configuration we see today.
The number of black holes is roughly three times larger than expected from the number of stars in the cluster, meaning that more than 20 percent of the total cluster mass is made up of black holes. Each black hole has a mass of about 20 times the mass of the Sun, and they formed in supernova explosions when the cluster was still very young.
In around a billion years, the cluster will dissolve completely, and what remains will consist entirely of black holes, orbiting the galactic center. This suggests that Palomar 5 is not unique, and other globular clusters will likely share the same fate. This discovery also offers confirmation that globular clusters may be excellent places to look for black holes that will eventually collide, as well as the elusive class of middleweight black holes.