Dynamics of supermassive black hole triples in the ROMULUS25 cosmological simulation

TL;DR

This study investigates the dynamical evolution and coalescence times of supermassive black hole triples from the ROMULUS25 simulation, revealing typical merger patterns and the formation of hierarchical triples.

Contribution

It provides high-resolution N-body simulations of SMBH triples from cosmological data, analyzing their evolution and coalescence times, which was not previously detailed.

Findings

Heaviest SMBH binaries merge within fractions of the Hubble time.

Lighter SMBHs often get ejected or form stable triples.

Hierarchical triples form in steep galactic nuclei density profiles.

Abstract

For a pair of supermassive black holes (SMBHs) in the remnant of a dual galaxy merger, well-known models exist to describe their dynamical evolution until the final coalescence accompanied by the emission of a low-frequency gravitational wave (GW) signal. In this article, we investigate the dynamical evolution of three SMBH triple systems recovered from the ROMULUS25 cosmological simulation to explore common dynamical evolution patterns and assess typical coalescence times. For this purpose, we construct initial conditions from the ROMULUS25 data and perform high-resolution gravitodynamical \N-body simulations. We track the orbital evolution from the galactic inspiral to the formation of hard binaries at sub-parsec separation and use the observed hardening rates to project the time of coalescence. In all cases, the two heaviest black holes form an efficiently hardening binary that merges within fractions of the Hubble time. The lightest SMBH either gets ejected, forms a stable hierarchical triple system with the heavier binary, forms a hardening binary with the previously merged binary's remnant, or remains on a wide galactic orbit. The coalescence times of the lighter black holes are thus significantly longer than for the heavier binary, as they experience lower dynamical friction and stellar hardening rates. We observe the formation of hierarchical triples when the density profile of the galactic nucleus is sufficiently steep.