A Black Hole Family Tree

Black holes can have messy family histories, and a recent massive merger challenged astronomers to sort out just how many generations of black holes were involved in the event. Recent research, however, may have untangled the lineages of the two progenitors.

A Massive Collision

Earlier this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the ripples in spacetime caused by the merger of two black holes somewhere in the distant universe. Though these detections have become increasingly common, this one in particular stood out among the more than 200 reported over the last decade. Named GW231123, this event must have caused more of a splash than a ripple since the two black holes that slammed together were unusually massive; both likely weighed more than 100 times the mass of the Sun. Not only that, each was also spinning unusually rapidly prior to the collision.

According to theory, black holes this large are not supposed to form on their own. When massive stars die, they can collapse into a black hole; but, when really massive stars die, the collapse is so violent that no material is left over to form the black hole. So, when researchers first measured the masses of the black holes involved in GW231123, they knew that the black holes involved couldn’t have formed directly from collapsed stars. Instead, they were likely second-generation black holes, meaning that they themselves formed from previous black hole–black hole mergers.

Merger Mystery

This result was celebrated as evidence of so-called “hierarchical formation,” in which black holes formed in dense environments swarm throughout their star clusters, merging with other black holes they encounter and growing ever larger.

However, there were a couple of complications with this picture. For one, theory predicts that it is challenging to form such rapidly spinning black holes from randomly oriented progenitors, since usually the spins wash out and leave the remnant rotating slowly. For another, the merging process tends to give the final black hole a “kick” that sends it flying rapidly away from the site of the impact, occasionally fast enough to leave the star cluster altogether, meaning second-generation black holes might not be able to merge into subsequent generations.

Born as a Pair

New research led by Jakob Stegmann, Max Planck Institute for Astrophysics, suggests a way around each of these problems. In the team’s model, the two black holes involved in the GW231123 merger didn’t form from isolated black holes in a cluster, but rather from binary stars embedded within that cluster. In this picture, two massive stars were born bound together, orbiting one another within a larger cluster of stars. Crucially for this scenario, the massive primordial stars have aligned spins — an expected outcome of massive binary star evolution.

After both stars in the binary collapsed into black holes, these black holes merged to form a rapidly spinning second-generation black hole. This second-generation black hole eventually encountered another black hole within the cluster and merged to produce the ripples in spacetime we observed as GW231123.

Since black holes born within binary systems are expected to have smaller kicks and faster spins, this scenario neatly explains the properties observed in GW231123. As LIGO keeps listening for more black hole mergers, hopefully similar events will provide more chances to untangle their complex family trees.

Citation

“Resolving Black Hole Family Issues Among the Massive Ancestors of Very High-Spin Gravitational-Wave Events like GW231123,” Jakob Stegmann et al 2025 ApJL 992 L226. doi:10.3847/2041-8213/ae0e5f