Is GW231123 a Hierarchical Merger?

The binary black hole merger GW231123 is both the most massive gravitational-wave event observed and has the highest component spins measured to date. The dimensionless spins of the more massive (primary) and less massive (secondary) black holes are measured to be χ1=0.90−0.19+0.10 and χ2=0.80−0.51+0.20 (90% credible intervals), respectively. Its large mass and extremal spins are challenging to explain through standard binary stellar physics, though a flurry of hypothetical scenarios have been proposed. Hierarchical assembly—i.e., mergers of black holes that are themselves formed from previous generations of mergers—is generally a promising way to explain massive and rapidly spinning black holes. Here, we investigate the possibility that GW231123 was assembled hierarchically in a dense star cluster as the merger of two second-generation black holes. Taking the inferred spin values at face value, we find that it is possible (p ≈ 5%) that a compact binary with component spins like GW231123 could form in a cluster from hierarchical assembly.