Perturber-Driven Dynamics of Supermassive Black Hole Binaries in Galaxy Merger
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 $N$-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 $N$-body code. A no-perturber control is compared with two matched suites in which $f_{\mathrm{target}}=0.1$ of the primary bulge mass is redistributed into equal-mass perturbers of $10^7,M_\odot$ ($\mu_{\mathrm{p}}\approx3.2\times10^{-3}$) and $10^8,M_\odot$ ($\mu_{\mathrm{p}}\approx3.2\times10^{-2}$), with four realisations per scenario. The control gives $\sigma_e\approx0.11$, consistent with the Poisson noise floor at this resolution. The $10^7,M_\odot$ case gives $\sigma_e\approx0.115$, indistinguishable from the control, whereas the $10^8,M_\odot$ case gives $\sigma_e\approx0.26$, a factor of $2.4$ above the floor, although 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 $\mu_{\mathrm{p}}$: the $10^7,M_\odot$ case remains diffusive, whereas the $10^8,M_\odot$ case approaches the near-impulsive regime. Because the expected perturber population in massive ellipticals lies mostly below this regime, perturber-driven eccentricity randomisation is unlikely to affect GWB-relevant MBHB mergers.