Snapshot: Black holes sprinkled in a globular star cluster

Globular clusters — spherical collections of ancient stars — are some of the oldest surviving inhabitants of our universe. For example, the cluster pictured above, NGC 6397, is roughly 13.4 billion years old, or nearly as old as the cosmos itself. And scientists thought NGC 6397 might be hiding a “missing link” in black hole evolution.

How petite stellar mass black holes evolve into gargantuan supermassive black holes, which lurk in the heart of nearly every known large galaxy, has flummoxed researchers for decades. One of leading theories is that an in-between, intermediate-mass black hole (IMBH) stage must exist; however, IMBHs have remained elusive for years. Recently, though, astronomers have found a few strong candidates, including 3XMM J215022.4-055108 and a gravitational wave signal from September 2020, even if they’ve failed to turn up definitive proof.

NGC 6397, located in the constellation Ara, hosts a surplus of invisible mass at its center. And for the past few years, Eduardo Vitral and Gary A. Mamon of the Institut d’Astrophysique de Paris thought that mass comes in the form of an IMBH. But by further studying the positions and velocities of the stars inside the cluster, Vitral and Mamon, discovered that the mass concentrated near the center of NGC 6397 is not actually point-like, as it should be if was all contained within an IMBH. Their research appeared in Astronomy & Astrophysics earlier this month.

Rather than an IMBH, they now think that a collection of smaller, yet still extremely dense, stellar corpses “sunk” to the center of the cluster over time. While these remnants are likely to include a number of white dwarfs and neutron stars, the researchers attribute the bulk of the mass to stellar-mass black holes.

And as an added bonus, the new research also suggests that the mass range of these star-sized black holes make them prime targets for the LIGO/Virgo collaboration to pick up should they collide sometime in the near future.