A set of black holes in a globular cluster

Black holes come in at least two sizes: small and large. Small black holes form from stars. When a large star reaches the end of its life, it usually ends in a supernova. The residual core then collapses under its own weight and forms a black hole or neutron star. Small stellar mass black holes are typically ten solar masses. Large black holes lurk in the centers of the galaxies. These supermassive black holes can be millions or billions of solar masses. They formed in the early universe and triggered the formation and evolution of galaxies around them.

Sizes of known black holes. Photo credit: NASA / JPL-Caltech

But maybe black holes will also come into the medium. These medium-mass black holes (IMBHs) could grow from the rapid mergers of star-mass black holes, spanning hundreds or thousands of solar masses. It has long been believed that medium-sized black holes could form in the hearts of globular clusters.

Globular clusters are dense star clusters that typically contain hundreds of thousands of stars. They formed in the early history of the universe and may be older than the galaxies themselves. Because globular clusters are tightly packed, their stars often have close encounters. These flyby tend to force smaller, less massive stars towards the outer regions of the clusters and more massive stars towards the center. It is therefore possible that stars in the core of a globular cluster merge to form a black hole of medium mass.

However, it is difficult to prove that IMBHs exist. Globular clusters are so dense that it is difficult to study their central regions. And when globular clusters contain intervening black holes, they’re too old to be very active that we can’t identify them through things like jets. As a result, we have few potential IMBH candidates. Recently, a team looked at the globular cluster NGC 6397, which is closest to Earth. What they found was quite surprising.

A gap in the orbital speed shows that there is no IMBH. Photo credit: Vitral & Mamon

Because the globular cluster is so dense, the team examined the line spectra of the cluster to determine the motion of the stars in the cluster. When stars orbit the core of the globular cluster, their line spectra shift due to the Doppler effect due to their movement relative to the earth. Even without observing individual stars, the team was able to obtain a statistical distribution of their movement.

If there was a medium-mass black hole in the center of NGC 6397, the stellar velocities should be evenly distributed, with the central stars orbiting faster than those on the periphery. Instead, the team found a dynamic gap in star motion. This suggests the idea that there is no central mass in the cluster. Instead, the central region is likely filled with white dwarfs, neutron stars, and black holes with stellar mass.

While many globular clusters may contain a medium-sized black hole, we now know that this is not always the case. This latest study shows that the dynamics of black hole formation are complex and we still have much to learn.

Reference: Vitral, Eduardo, and Gary A. Mamon. “Does NGC 6397 contain a medium-mass black hole or a more diffuse inner sub-cluster?” Astronomy & Astrophysics 646 (2021): A63.

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