Resolving the Complex Evolution of a Supermassive Black Hole Triplet in a Cosmological Simulation

TL;DR

This paper presents a detailed cosmological simulation of a triple supermassive black hole system, capturing complex interactions and their effects on galaxy evolution, which are difficult to model with simpler methods.

Contribution

The study introduces an advanced simulation code, KETJU, capable of accurately modeling the dynamics of multiple SMBHs during galaxy mergers, including relativistic effects and stellar interactions.

Findings

One SMBH binary merges after ~3 Gyr.

A third SMBH is ejected to a kiloparsec orbit.

The simulation shows SMBH interactions influence stellar populations.

Abstract

We present here a self-consistent cosmological zoom-in simulation of a triple supermassive black hole (SMBH) system forming in a complex multiple galaxy merger. The simulation is run with an updated version of our code KETJU, which is able to follow the motion of SMBHs down to separations of tens of Schwarzschild radii while simultaneously modeling the large-scale astrophysical processes in the surrounding galaxies, such as gas cooling, star formation, and stellar and AGN feedback. Our simulation produces initially a SMBH binary system for which the hardening process is interrupted by the late arrival of a third SMBH. The KETJU code is able to accurately model the complex behavior occurring in such a triple SMBH system, including the ejection of one SMBH to a kiloparsec-scale orbit in the galaxy due to strong three-body interactions as well as Lidov-Kozai oscillations suppressed by relativistic precession when the SMBHs are in a hierarchical configuration. One pair of SMBHs merges $\sim 3\,\mathrm{Gyr}$ after the initial galaxy merger, while the remaining binary is at a parsec-scale separation when the simulation ends at redshift $z=0$. We also show that KETJU can capture the effects of the SMBH binaries and triplets on the surrounding stellar population, which can affect the binary merger timescales as the stellar density in the system evolves. Our results demonstrate the importance of dynamically resolving the complex behavior of multiple SMBHs in galactic mergers, as such systems cannot be readily modeled using simple orbit-averaged semi-analytic models.