# 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/en/arxiv-id/2601.07908v1/

## Key points

- Study focuses on hierarchical mergers in dense stellar clusters to fill gap in black hole masses caused by pair-instability process
- Hierarchical mergers show unique spin and tilt characteristics compared to first-generation black holes, indicating distinct evolutionary pathways
- Introduction of astrophysically-motivated model capturing dominant spin dynamics for hierarchical mergers
- Compelling evidence for population transition above 45 solar masses aligning with onset of pair-instability gap
- Identification of peak around 15 solar masses as global maximum in hierarchical merger rate
- Existence of low- and high-mass subpopulations of higher-generation black holes from near-solar-metallicity and metal-poor star clusters
- Reinforcement of detailed mass-dependent substructure within binary black hole population's spin distribution
- Acknowledgment of valuable discussions with colleagues and funding support from various sources including NSF Graduate Research Fellowship and Brinson Foundation grants
- Recognition of computational resources provided by LIGO Laboratory and data obtained from Gravitational Wave Open Science Center
- Study sheds light on interplay between stellar evolution processes and black hole formation through hierarchical mergers within dense stellar clusters, contributing significantly to understanding black hole populations and their evolutionary history

## Summary

In their study titled "Signatures of a subpopulation of hierarchical mergers in the GWTC-4 gravitational-wave dataset," Cailin Plunkett, Thomas Callister, Michael Zevin, and Salvatore Vitale explore the role of repeated black-hole mergers in dense stellar clusters as a mechanism to fill the predicted gap in black hole masses caused by the pair-instability process. These hierarchical mergers exhibit unique spin and tilt characteristics compared to first-generation black holes, indicating distinct evolutionary pathways. The researchers introduce an astrophysically-motivated model that considers both effective inspiral and precessing spins, capturing the dominant spin dynamics expected for hierarchical mergers. Through their analysis, they provide compelling evidence for a population transition above approximately 45 solar masses, aligning with the anticipated onset of the pair-instability gap. Additionally, they identify a peak around 15 solar masses, which they interpret as the global maximum in the hierarchical merger rate. Furthermore, their findings suggest the existence of low- and high-mass subpopulations of higher-generation black holes, implying contributions from both near-solar-metallicity and metal-poor star clusters to the hierarchical merger population. This discovery reinforces previous indications of detailed mass-dependent substructure within the binary black hole population's spin distribution. The authors acknowledge valuable discussions with colleagues and express gratitude for funding support from various sources including the National Science Foundation Graduate Research Fellowship and grants from organizations like the Brinson Foundation. They also recognize computational resources provided by LIGO Laboratory and data obtained from the Gravitational Wave Open Science Center. Overall, this study sheds light on the intricate interplay between stellar evolution processes and black hole formation through hierarchical mergers within dense stellar clusters. The refined analysis presented here contributes significantly to our understanding of black hole populations and their evolutionary history in astrophysics research.