The effect of triaxiality on the dynamics of triple supermassive black holes in a cosmological context

TL;DR

The paper addresses how triaxial galactic potentials influence the long‑term dynamics and coalescence of triple SMBHs in a cosmological setting. It advances the study with high‑resolution gravito‑dynamical N‑body simulations that combine BONSAI2 for the early inspiral and $\varphi$‑GPU for close three‑body interactions, incorporating Post‑Newtonian corrections up to 2.5PN. The main finding is that the two most massive SMBHs form a hard inner binary that typically coalesces within a fraction of a Hubble time, while the third BH is captured into a stable hierarchical triple or remains on a wide orbit; three‑body encounters can exchange binary members but do not prevent the hierarchical outcome. This suggests that triaxiality does not strongly modify the final triple SMBH configuration or broad merger timescales, with implications for gravitational‑wave event rates and SMBH assembly in hierarchical structure formation.

Abstract

The hierarchical nature of galaxy formation in the $Λ$CDM framework often leads to multiple supermassive black holes (SMBHs) in the galactic nuclei. The timescale over which galaxies merge, plays a crucial role in shaping the dynamical evolution and the merger dynamics of their central SMBHs. While binary SMBH evolution is well studied, the long-term dynamics of triple SMBH systems, particularly in non-spherical potentials, remain less understood. We investigate the role of triaxiality in the evolution and dynamics of triple SMBHs with initial conditions drawn from the ROMULUS25 cosmological simulation, using high-resolution gravitodynamical N-body simulations. We explore different orbital configurations and host shapes, tracking the evolution from galactic inspiral to hard binary formation at sub-parsec scales. In all cases, the two most massive SMBHs form a rapidly hardening binary that coalesces within a fraction of a Hubble time, while the third forms a stable hierarchical triple system with the heavier binary, or remains on a wide orbit.

Figures (2)

• Figure 1: Orbital evolution of the three SMBHs where the three BH1,BH2 & BH3 are color-coded as red, blue and green respectively showing the continuous exchange of the SMBHs due to three body encounters.
• Figure 2: Black hole orbital parameters across the different simulations. The color coding refers to different SMBH pairs. The red lines refer to the orbital parameters around the inner BH1-BH2 binary (two most massive BHs). Background colors mark the codes used during the particular simulation period, light gray for Bonsai2, white for $\varphi$-GPU. The left panels show the separation of different SMBH pairs and the right panels show the evolution of the inverse semi-major axis. The black dotted, dashed, and solid lines, mark the influence radius, hardening radius, and the factor 1000 of the Schwarzschild radius ($R_s$) of BH1 respectively.