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

TL;DR

GW231123 presents an exceptionally massive BBH merger with $M_{ m tot}\sim190$–$265\,M_\odot$ and high spins, challenging the pair-instability mass gap. The authors apply population-informed priors based on GWTC-3 spin subpopulations to test formation channels and find that both components are best described as higher-generation BHs, likely assembled from roughly $6$ and $4$ first-generation BHs for the primary and secondary, respectively. This supports a hierarchical-merger origin and implies that repeated mergers can yield very massive BHs, possibly IMBHs, with AGN-disk gas dynamics proposed as an efficient hardening mechanism. Overall, the work demonstrates how incorporating population-level spin/mass information can sharpen interpretations of individual events and inform the astrophysical pathways shaping the BBH landscape.

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 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 multi-generation ($>$2G) merger remnants, and plausibly originated 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 friction in the disks of active galactic nuclei.

Figures (5)

• Figure 1: Spin magnitude v.s. component mass distribution of GW231123 ( reweighed by HS-HS prior) compared to those of BBH events in GWTC-3 informed by population model of 2024PhRvL.133e1401L. The red and blue contours denote the primary and secondary components of GW231123. The green shaded regions show the $90\%$ credible regions for component BHs in the GWTC-3 population, and the black (orange) points mark the mean component masses for the primary (secondary) BHs in those events.
• Figure 2: Comparison of final remnant spins (left panel) and GW recoil kick velocities(right panel) for mergers with BHs of different generations. The left panel shows the corresponding distributions of the remnants' final spin magnitudes, with shaded areas comparing the component spin magnitudes of GW231123, reweighed with HS-HS priors. The right panel shows the kick velocity distributions, indicating that hierarchical mergers involving higher-generation component BHs produce significantly larger kicks than 1G+1G mergers. The 1G+1G, nG+1G, and nG+nG BBHs samples are drawn from the posterior population distribution in 2024PhRvL.133e1401L.
• Figure 3: Mass distributions of merger remnants formed from first-generation BHs in dynamical formation channels, compared to the component masses of GW231123, reweighed with HS-HS prior. The first-generation BH mass distribution is taken from 2024ApJ...977...67L. The pink and grey shaded regions indicate GW231123's primary and secondary mass distributions, which align with mass distribution of remnants assembled from $\sim6$ (green) and $\sim4$ (blue) first-generation BHs, respectively. A self-consistency check comparing these model predictions to the observed higher-generation BH population is provided in Appendix \ref{['app:check']} (see Figure \ref{['fig:check-dist']}).
• Figure 4: Posterior distributions of component masses, spin magnitudes, and spin orientations of GW231123 under different priors.
• Figure 5: Mass distributions of the subpopulations comparing to the remnants of mergers drawn from the potential first-generation subpopulation. Top: the first-generation BHs are drawn from the potential dynamical first-generation subpopulation (green region), the subpopulation in purple are potentially associated with field evolution channels. Bottom: the first-generation BHs are drawn from the low-spin subpopulation (orange region), which is likely the mixture of dynamical and field channels. Note that each distribution of subpopulation is normalized, and the shaded regions are for $90\%$ credible levels.