The orbital eccentricity of massive black hole binaries (MBHBs) at binary formation shapes the stochastic gravitational-wave background (GWB) detectable by pulsar timing arrays (PTAs). Previous í µí±-body simulations show large run-to-run scatter in this quantity, dominated by Poisson noise, raising the question of whether physical substructure adds genuine astrophysical stochasticity. We test this with high-resolution re-simulations of a major merger from IllustrisTNG100-1, evolved with the Griffin í µí±-body code. A no-perturber control is compared with two matched suites in which í µí±“ target = 0.1 of the primary bulge mass is redistributed into equal-mass perturbers of 10 7 M ⊙ (í µí¼‡ p ≈ 3.2 × 10 −3) and 10 8 M ⊙ (í µí¼‡ p ≈ 3.2 × 10 −2), with four realisations per scenario. The control gives í µí¼Ž í µí±’ ≈ 0.11, consistent with the Poisson noise floor at this resolution. The 10 7 M ⊙ case gives í µí¼Ž í µí±’ ≈ 0.115, indistinguishable from the control in its scatter, whereas the 10 8 M ⊙ case gives í µí¼Ž í µí±’ ≈ 0.26, a factor of 2.4 above the floor, though statistically marginal given only four realisations. This excess scatter coincides with larger event-aligned residuals in orbital energy and angular momentum and stronger torque spikes, consistent with near-impulsive perturber–MBHB encounters. In binary–single scattering theory the transition is set by the perturber–MBHB mass ratio í µí¼‡ p : the 10 7 M ⊙ case remains diffusive while the 10 8 M ⊙ case approaches the near-impulsive regime. As the expected perturber population in massive ellipticals lies mostly below this regime, perturber-driven eccentricity randomisation is unlikely to matter for GWB-relevant MBHB mergers.
- Perturber-Driven Dynamics of Supermassive Black Hole Binaries in Galaxy Merger
- Alessia Gualandris (Author) - University of Surrey, School of Maths and PhysicsJulian Jun Yen Chan (Author) - University of Surrey, School of Maths and PhysicsJustin Inglis Read - University of Surrey, School of Maths and PhysicsWalter Dehnen (Author) - Heidelberg University
- Monthly Notices of the Royal Astronomical Society, Vol.550(4), stag1374
- Oxford University Press
- 12
- 28/07/2026
- 08/2026
- 14/07/2026
- The dawn of low frequency Gravitational Wave astronomy, RD6198, Science and Technology Facilities Council (STFC)ST/Y002385/1, Science and Technology Facilities Council (United Kingdom, Swindon) - STFC
- AG acknowledges support from grant ST/Y002385/1.
- 991145395502346
- © The Author(s) 2026. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
- School of Maths and Physics
- English
- Journal article
- The data underlying this article will be shared on reasonable request to the corresponding author. The analysis scripts used to process the simulation outputs and generate the figures will also be shared on reasonable request. This work made use of the griffin N-body code and standard python scientific libraries, including numpy, scipy, pandas, h5py, matplotlib, and scienceplots.