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

TL;DR

This study examines how the shape of galaxy potentials influences the long-term dynamics of triple supermassive black holes, revealing that the most massive pair quickly merges while the third can remain in a stable orbit.

Contribution

It provides new insights into the role of triaxial galaxy shapes on the evolution and stability of triple SMBH systems using cosmological simulation data.

Findings

Most massive SMBHs form a rapidly merging binary.

Third SMBH often remains in a wide or stable orbit.

Triaxiality affects the dynamical evolution of triples.

Abstract

The hierarchical nature of galaxy formation in the $\Lambda$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.