Black hole populations and hierarchical growth across the AGN parameter space
Abstract
Active galactic nuclei (AGNs) have been proposed as efficient environments for the formation of binary black holes (BBHs). We present an updated semi-analytical framework for BBH formation and evolution in AGN disks, following the capture, migration, pair-up, gas-driven hardening, binary--single encounters, and merger of stellar-origin black holes. We systematically explore the dependence of the resulting BBH merger population on the main AGN parameters, namely the supermassive black hole mass M•, the Eddington ratio λ•, and the disk viscosity parameter α, and construct an intrinsic BBH population by weighting the simulations according to observed low-redshift AGN properties. We find that AGN disks can produce repeated mergers and build a high-mass tail extending beyond the pair-instability mass gap and into the intermediate-mass range. Hierarchical growth is more efficient in lower-viscosity disks, with α=0.01, while higher-viscosity disks suppress the formation of massive remnants. The merger efficiency generally increases with λ•, but its dependence on M• is non-trivial. The AGN-assisted BBH population is characterized by increasingly unequal mass ratios at high primary mass, a correlation between primary mass and |χeff|, and an effective-spin distribution that depends strongly on the fraction of binaries born in prograde or retrograde configurations. We find that the AGN channel can reproduce systems broadly consistent with the massive BBH events GW190521 and GW231123. We test several variations of the physical model, including different formalisms for migration torques, gas hardening, and three-body encounters. The general properties of the population are robust across these variations, with the high-mass tail and spin signatures persisting in all cases except when gas hardening is switched off.
Versions
Related
- Physics-based phenomenological modeling of binary black hole hierarchical formation 2: Autodifferentiable functional inference of hierarchical compact-binary populations
- An adaptive parameter optimization method for astronomical image alignment using Bayesian optimization. I. A hierarchical search strategy for FWHM and SNR
- Theory-constrained hierarchical inference of primordial black holes in f(Q, T) gravity with GWTC-3
- Aligned Hierarchical Black Hole Mergers in Active-Galactic-Nuclei Disks Revealed by GWTC-4
- Parameter Estimation for Eccentric Supermassive Black Hole Binaries with Pulsar Timing Arrays
BibTeX
@misc{vaccaro2026black,
title = {{Black hole populations and hierarchical growth across the AGN parameter space}},
author = {Maria Paola Vaccaro and Michela Mapelli and Alessandro Alberto Trani and Boyuan Liu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.10823},
eprint = {2606.10823},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.10823},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Black hole populations and hierarchical growth across the AGN parameter space
AU - Maria Paola Vaccaro
AU - Michela Mapelli
AU - Alessandro Alberto Trani
AU - Boyuan Liu
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2606.10823
UR - https://arxiv.org/abs/2606.10823
AB - Active galactic nuclei (AGNs) have been proposed as efficient environments for the formation of binary black holes (BBHs). We present an updated semi-analytical framework for BBH formation and evolution in AGN disks, following the capture, migration, pair-up, gas-driven hardening, binary--single encounters, and merger of stellar-origin black holes. We systematically explore the dependence of the resulting BBH merger population on the main AGN parameters, namely the supermassive black hole mass M_•, the Eddington ratio λ_•, and the disk viscosity parameter α, and construct an intrinsic BBH population by weighting the simulations according to observed low-redshift AGN properties. We find that AGN disks can produce repeated mergers and build a high-mass tail extending beyond the pair-instability mass gap and into the intermediate-mass range. Hierarchical growth is more efficient in lower-viscosity disks, with α=0.01, while higher-viscosity disks suppress the formation of massive remnants. The merger efficiency generally increases with λ_•, but its dependence on M_• is non-trivial. The AGN-assisted BBH population is characterized by increasingly unequal mass ratios at high primary mass, a correlation between primary mass and |χ_eff|, and an effective-spin distribution that depends strongly on the fraction of binaries born in prograde or retrograde configurations. We find that the AGN channel can reproduce systems broadly consistent with the massive BBH events GW190521 and GW231123. We test several variations of the physical model, including different formalisms for migration torques, gas hardening, and three-body encounters. The general properties of the population are robust across these variations, with the high-mass tail and spin signatures persisting in all cases except when gas hardening is switched off.
ER -