# Signatures of a subpopulation of hierarchical mergers in the GWTC-4 gravitational-wave dataset

**Authors:** Cailin Plunkett, Thomas Callister, Michael Zevin, Salvatore Vitale  
**arXiv:** [2601.07908](https://arxiv.org/abs/2601.07908)  
**Date:** 2026-01-12  
**Summary page:** https://www.summarizepaper.com/fr/arxiv-id/2601.07908v1/

## Abstract

Repeated black-hole mergers in dense stellar clusters are a plausible mechanism to populate the predicted gap in black hole masses due to the pair-instability process. These hierarchical mergers carry distinct spin and tilt features relative to first-generation black holes, for which previous studies have found evidence at a population level by interpreting features in the effective inspiral spin domain. We introduce an astrophysically-motivated model in the joint space of effective inspiral and precessing spins, which captures the dominant spin dynamics expected for hierarchical mergers. We find decisive evidence both for a population transition above $\sim 45M_\odot$, consistent with the anticipated onset of the pair-instability gap, as well as a peak at $\sim 15 M_\odot$, which we interpret as the global peak in the hierarchical merger rate. The existence of low- and high-mass subpopulations of higher-generation black holes suggests the contribution of both near-solar-metallicity and metal-poor star clusters to the hierarchical merger population. Our results reinforce the growing evidence for detailed, mass-dependent substructure in the spin distribution of the binary black hole population.