GW231123: a product of successive mergers from $\sim 10 $ stellar-mass black holes

Abstract: GW231123 is an exceptionally massive binary black hole (BBH) merger with unusually high component spins. Such extreme properties challenge conventional stellar evolution models predicting a black hole mass gap due to pair-instability supernovae. We analyze GW231123 using population-informed priors on BH mass and spin distributions to test possible formation scenarios: first-generation stellar collapse, hierarchical (multi-generation) mergers, and primordial origin. Our analysis strongly prefers scenarios where at least one component is a higher-generation BH. Both components are favored to have high spins, which rules out scenarios in which they are both first-generation (low spin) or primordial (nearly non-spin). We conclude that GW231123 is a hierarchical merger, with components plausibly originate from the successive mergers of $\sim 6$ and $\sim 4$ first-generation BHs, respectively. This suggests that repeated mergers can be frequent and even more massive intermediate-mass black holes may be produced. Thus mechanisms that can efficiently harden the BBHs' orbits are required, e.g., gas dynamical fraction in the disks of active galactic nucleus.