Dynamical evolution timescales for the triple supermassive black hole system in NGC 6240

TL;DR

This study models the dynamical evolution of the triple supermassive black hole system in NGC 6240, revealing the formation timescales, ZLK oscillations, and conditions for black hole mergers through detailed Newtonian and post-Newtonian simulations.

Contribution

It provides the first detailed long-timescale N-body simulations of the NGC 6240 SMBH system, including ZLK oscillations and merger conditions with GPU acceleration.

Findings

Bound SMBH system forms at ~3.6 Myr.

Hierarchical triple system forms at ~18 Myr.

ZLK oscillations observed in most models.

Abstract

Based on the available observational data from the literature, we analysed the dynamics of the NGC 6240 galaxy central supermassive black hole (SMBH) system. For the dynamical modelling of this triple SBMH system, we used the massively parallel and GPU accelerated phi-GPU direct summation N-body code. Following a long-timescale modelling of the triple system, we carried out a very detailed time output analysis of the von Zeipel-Lidov-Kozai (ZLK) oscillations for the black holes. According to our Newtonian simulation results, for all models and randomisations, the bound system from S1+S2 components formed at ~3.6 Myr. The formation of the bound hierarchical triple system S+N occurred at ~18 Myr. Over the course of these Newtonian simulations of the evolution of the triple SMBH system and the surrounding environment in NGC 6240, ZLK oscillations were detected (in most cases) for the binary components. The inclination angle between the orbital angular momentum of binary components aptly coincides with the theoretical calculations of the ZLK mechanism. In our set of randomised 15 Newtonian $N$-body dynamical galaxy models in 13 systems, we were able to detect a ZLK mechanism. In contrast, our extra few-body post-Newtonian runs (for one randomisation case) show it is only for the large inner binary initial eccentricity (in our case >0.9 that we are able to observe the possibility of the inner binary merging, due to the post-Newtonian energy radiation effects. For the lower eccentricity cases, the test runs show no sign of possible merging or any ZLK oscillations in the system.