GW231123: Likely a Product of Successive Mergers from ∼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 (BH) mass gap due to pair-instability supernovae. We test possible formation scenarios for GW231123 using population-informed priors on BH spin distributions, in light of population properties built on the previous (GWTC-3) data. Our analysis shows that GW231123 belongs to the high-spin subpopulation that is naturally interpreted as hierarchical BBH mergers. By comparing the spin magnitudes and component masses of GW231123 to those of the remnants of previous mergers, we show that both components of GW231123 are multigeneration (>2G) merger remnants, and plausibly originated from the successive mergers of ∼6 and ∼4 first-generation BHs, respectively. This suggests that repeated mergers can be frequent, and even more massive intermediate-mass BHs may be produced. Thus, mechanisms that can efficiently harden the BBHs’ orbits are required, e.g., gas dynamical friction in the disks of active galactic nuclei.