2026ApJ...999..236P

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 ${\chi }_{1}=0.9{0}_{-0.19}^{+0.10}$ and ${\chi }_{2}=0.8{0}_{-0.51}^{+0.20}$ (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.