Building the Black Hole Family Tree

In 2019, astronomers observed an unusual gravitational chirp. Known as GW190521, it was the last scream of gravitational waves as a black hole of 66 solar masses merged with a black hole of 85 solar masses to become a 142 solar mass black hole. The data were consistent with all the other black hole mergers we’ve observed. There was just one problem: an 85 solar mass black hole shouldn’t exist.

All the black hole mergers we’ve observed involve stellar mass black holes. These form when a massive star explodes as a supernova and its core collapses to become a black hole. An old star needs to be at least ten times the mass of the Sun to become a supernova, which can create a black hole of about 3 solar masses. Larger stars can create larger black holes, up to a point.

The first generation of stars in the cosmos were likely hundreds of solar masses. For a star above 150 solar masses or so, the resulting supernova would be so powerful that its core would undergo what is known as pair-instability. Gamma rays produced in the core would be so intense they decay into an electron-positron pair. The high-energy leptons would then rip apart the core before gravity could collapse it. To overcome the pair-instability, a progenitor star would need a mass of 300 Suns or more. This means that the mass range of stellar black holes has a “pair-instability gap.” Black holes from 3 solar masses to about 65 solar masses would form from regular supernovae, and black holes above 130 solar masses could form from stellar collapse, but black holes between 65-130 solar masses shouldn’t exist.

For GW190521, the 66 solar mass black hole is close enough to the limit that it likely formed from a single star. The 85 solar mass black hole, on the other hand, is smack-dab in the middle of the forbidden range. Some astronomers have argued that the larger black hole might have formed from a hypothetical boson star known as a Proca star, but if that’s true, then GW190521 is the only evidence that Proca stars exist. More likely, the 85 solar mass black hole formed from the merger of two smaller black holes, making GW190521 a staged merger. The difficulty with that idea is that black hole mergers are often asymmetrical, in a way that the resulting black hole is kicked out of its region of origin. Multiple black hole mergers would only occur under certain circumstances, which is where a new study in The Astrophysical Journal comes in.

The authors looked at how the mass, spin, and motion of a merging black hole pair determine the mass, spin, and recoil velocity of the resulting black hole. By creating a statistical distribution of outcomes, the team could then work backwards. Given the mass, spin, and velocity of a “forbidden” black hole relative to its environment, what were the properties of its black hole ancestors? When the authors applied this to the progenitors of GW190521, they found that the only possible ancestors would have given a relatively large recoil velocity. This means that the merger must have occurred within the region of an active galactic nucleus, where the gravitational well would be strong enough to hold the system together.

This work has implications for what are known as intermediate mass black holes (IMBHs), which can have masses of hundreds or thousands of Suns. It has been thought that IMBHs form within globular clusters, but if the recoil velocities of black hole mergers are large, this would be unlikely. As this study shows, GW190521 could not have occurred in a globular cluster.

Reference: Araújo-Álvarez, Carlos, et al. “Kicking Time Back in Black Hole Mergers: Ancestral Masses, Spins, Birth Recoils, and Hierarchical-formation Viability of GW190521.” The Astrophysical Journal 977.2 (2024): 220.

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