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

TL;DR

This study explores the origin of GW231123, a gravitational wave event with black hole masses in the pair-instability gap, suggesting a hierarchical merger origin in dense globular clusters supported by cosmological simulations.

Contribution

First self-consistent simulation framework coupling galaxy formation and binary evolution models to compare formation channels of GW231123-like events.

Findings

Both population synthesis codes can produce compatible black holes.

Isolated binary evolution cannot explain the observed merger redshift.

Hierarchical mergers in dense globular clusters naturally reproduce observed properties.

Abstract

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 Letter, we investigate its origin by coupling the galaxy formation model GAMESH with the cluster population synthesis code RAPSTER, and with two distinct binary population synthesis codes (SEVN and BSEEMP). This framework allows 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. We find that, although both population synthesis codes can in principle produce black holes compatible with GW231123, isolated binary evolution fails to reproduce the inferred merger redshift. In SEVN, massive black hole binaries form with semi-major axes > 10^3 Rsun , preventing coalescences within a Hubble time. In BSEEMP, 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 instead 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 find a local merger rate density of 0.78 Gpc^{-3} yr^{-1}, with a peak at z = 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.