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

TL;DR

This study investigates how massive, highly spinning BHs can form in dense star clusters, assessing accretion from BH–star collisions as an alternative to hierarchical mergers. It employs 12 Cluster Monte Carlo simulations coupled with COSMIC population synthesis to model BH formation, mergers, and accretion across varied accretion efficiencies and cluster properties. The results show accretion can produce BHs up to $\sim 200\,M_\odot$, with coherent episodes yielding $\chi \gtrsim 0.7$ for masses up to $\sim 150\,M_\odot$, while heavier BHs formed through multiple stochastic accretion events spin down to $\chi \lesssim 0.4$. BBH mergers from this channel can contribute up to $\sim 10\%$ of detectable events and tend to have near-equal masses, offering a distinct signature from hierarchical mergers which favor unequal mass ratios. This accretion-driven pathway enables low-escape-speed clusters (e.g., globular clusters) to produce highly spinning BBHs with components in or above the mass gap, providing a natural formation route to GW231123-like systems.

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 $\lesssim 10^6\,M_\odot$ 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. Using $N$-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 $\sim200\,M_\odot$, with spins set by the details of the growth history. BHs up to about $150\,M_\odot$ can reach dimensionless spins $χ\gtrsim 0.7$ via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down to $χ\lesssim 0.4$. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to $\sim10\%$ 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.

Figures (1)

• Figure 1: Total mass of BBH mergers as a function of merger time without accretion ($f_{\rm acc} = 0$, blue) and with accretion ($f_{\rm acc} = 1$, orange) in globular clusters with an escape speed of about $50\,\rm{km\,s^{-1}}$. Solid and open circles indicate in-cluster and ejected mergers, respectively. Triangles and circles denote models with initial virial radii of $r_{\rm v} = 1.0$ pc and $r_{\rm v} = 0.5$ pc, respectively. The shaded orange band marks the mass range of GW231123 GW231123, with the dashed line indicating its median total mass. The dotted gray lines indicate the redshift $z=1$, assuming present-day cluster ages of 10 Gyr and 12 Gyr.