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1. Hints of Spin-magnitude Correlations and a Rapidly Spinning Subpopulation of Binary Black Holes

   https://doi.org/10.3847/1538-4357/ae1574  

   Hussain, Asad; Isi, Maximiliano; Zimmerman, Aaron (December 2025, The Astrophysical Journal)

   Abstract The complex astrophysical processes leading to the formation of binary black holes and their eventual merger are imprinted on the spins of the individual black holes. We revisit the astrophysical distribution of those spins based on gravitational waves from the third Gravitational-wave Transient Catalog, looking for structure in the two-dimensional space defined by the dimensionless spin magnitudes of the heavier (χ₁) and lighter (χ₂) component black holes. We find support for two distinct subpopulations with greater than 95% credibility. The dominant population is made up of black holes with small spins, preferringχ₁≈ 0.2 for the primary andχ₂≈ 0 for the secondary; we report signs of an anticorrelation betweenχ₁andχ₂, as well as evidence against a subpopulation of binaries in which both components are nonspinning. The subdominant population consists of systems in which both black holes have relatively high spins and contains $20_{-18}^{+18}\%$ of the binaries. The binaries most likely to belong in this subpopulation are massive and slightly more likely to have spin orientations aligned with the orbital angular momentum—potentially consistent with isolated binary formation channels capable of producing large spins, like chemically homogeneous evolution. This hint of a rapidly spinning subpopulation hinges on GW190517, a binary with large, well-measured spins. Our results, enabled by novel hierarchical inference methods, represent a first step towards more descriptive population models for black hole spins and will be strengthened or refuted by the large number of gravitational-wave detections expected in the next several years. 

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2. Limits on Hierarchical Black Hole Mergers from the Most Negative χ _eff Systems

   https://doi.org/10.3847/2041-8213/ac86c4  

   Fishbach, Maya; Kimball, Chase; Kalogera, Vicky (August 2022, The Astrophysical Journal Letters)

   Abstract It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes χ ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, χ eff < −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitational-wave (GW) detectors have yet to observe a BBH system with χ eff ≲ −0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with χ eff < −0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90% credibility). Even among BBH systems with primary masses m 1 = 60 M ⊙ , the HM fraction is less than 69%, which may constrain the location of the pair-instability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with χ eff < −0.3, we can conclude that the HM fraction is smaller than 2.5 − 2.2 + 9.1 % . 

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3. Beyond Hierarchical Mergers: Accretion-driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters

   https://doi.org/10.3847/2041-8213/ae1eeb  

   Kıroğlu, Fulya; Kremer, Kyle; Rasio, Frederic A (November 2025, The Astrophysical Journal Letters)

   Abstract GW231123, the most massive binary black hole (BBH) merger detected by LIGO–Virgo–KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called “hierarchical merger” channel), most star clusters with masses ≲10⁶M_⊙have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH–star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. UsingN-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process, and we find that accretion can drive BH masses up to at least ∼200M_⊙, with spins set by the details of the growth history. BHs up to about 150M_⊙can reach dimensionless spinsχ ≳ 0.7 via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down toχ ≲ 0.4. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to ∼10% of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems. 

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4. Black Hole Accretion and Spin-up through Stellar Collisions in Dense Star Clusters

   https://doi.org/10.3847/1538-4357/ada26b  

   Kıroğlu, Fulya; Kremer, Kyle; Biscoveanu, Sylvia; González_Prieto, Elena; Rasio, Frederic A (January 2025, The Astrophysical Journal)

   Abstract Dynamical interactions in dense star clusters could significantly influence the properties of black holes, leaving imprints on their gravitational-wave signatures. While previous studies have mostly focused on repeated black hole mergers for spin and mass growth, this work examines the impact of physical collisions and close encounters between black holes and (noncompact) stars. Using Monte CarloN-body models of dense star clusters, we find that a large fraction of black holes retained upon formation undergo collisions with stars. Within our explored cluster models, the proportion of binary black hole mergers affected by stellar collisions ranges from 10%–60%. If all stellar-mass black holes are initially nonspinning, we find that up to 40% of merging binary black holes may have components with dimensionless spin parameterχ ≳ 0.2 because of prior stellar collisions, while typically about 10% have spins nearχ = 0.7 from prior black hole mergers. We demonstrate that young star clusters are especially important environments, as they can produce collisions of black holes with very massive stars, allowing for significant spin-up of the black holes through accretion. Our predictions for black hole spin distributions from these stellar collisions highlight their sensitivity to accretion efficiency, underscoring the need for detailed hydrodynamic calculations to better understand the accretion physics following these interactions. 

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5. Investigating the formation channel of GW231123: Population III stars or hierarchical mergers?

   https://doi.org/10.1051/0004-6361/202660148  

   Angeloni, Federico; Kritos, Konstantinos; Schneider, Raffaella; Berti, Emanuele; Graziani, Luca; Torniamenti, Stefano; Mapelli, Michela; Tanikawa, Ataru (June 2026, Astronomy & Astrophysics)

   The gravitational wave event GW231123, with component black hole masses lying within or above the pair-instability mass gap, poses a significant challenge to current stellar evolution models. In this work, we describe how we investigated its origin by coupling the galaxy formation modelGAMESHwith the cluster population synthesis codeRAPSTERand using two distinct binary population synthesis codes,SEVNandBSEEMP. This framework allowed us, for the first time, to reconstruct the life cycle of GW231123-like candidates within the same cosmological simulation, enabling a self-consistent comparison between different formation channels. Although both population synthesis codes can in principle produce black holes compatible with GW231123, we find that isolated binary evolution fails to reproduce the inferred merger redshift. InSEVN, massive black hole binaries form with semi-major axes > 10³R_⊙, preventing coalescences within a Hubble time. InBSEEMP, candidates arise only at extremely low metallicities (Z ≈ 10⁻¹⁰), which contribute negligibly to the star formation rate density in our overdense simulated volume. Our results therefore strongly support a dynamical hierarchical origin. The observed black hole masses are naturally reproduced through successive mergers in dense globular clusters. The high dimensionless spins reported by the LIGO-Virgo-KAGRA Collaboration are consistent with this hierarchical population. We found a local merger rate density of 0.78 Gpc⁻³yr⁻¹, with a peak atz = 4 − 6, tracing the maximum formation rate of globular clusters in metal-poor environments (Z ≈ 0.006). Overall, GW231123 may represent a benchmark event for a robust population of hierarchical black holes formed in the early Universe. 

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