The influence of Massive Black Hole Binaries on the Morphology of Merger Remnants

TL;DR

This study uses N-body simulations to explore how massive black hole binaries influence the shape and stellar dynamics of galaxy merger remnants, revealing they tend to produce nearly oblate structures and slightly eject stars.

Contribution

It provides new insights into the morphological effects of MBH binaries on galaxy remnants, especially their tendency to make systems oblate and influence stellar anisotropy.

Findings

Remnants with MBH binaries are nearly oblate and aligned with the merger plane.

MBH binaries slightly eject stars, about 1%, from the galactic nucleus.

Remnant morphology is not strongly linked to binary hardening rate.

Abstract

Massive black hole (MBH) binaries, formed as a result of galaxy mergers, are expected to harden by dynamical friction and three-body stellar scatterings, until emission of gravitational waves (GWs) leads to their final coalescence. According to recent simulations, MBH binaries can efficiently harden via stellar encounters only when the host geometry is triaxial, even if only modestly, as angular momentum diffusion allows an efficient repopulation of the binary loss cone. In this paper, we carry out a suite of N-body simulations of equal-mass galaxy collisions, varying the initial orbits and density profiles for the merging galaxies and running simulations both with and without central MBHs. We find that the presence of an MBH binary in the remnant makes the system nearly oblate, aligned with the galaxy merger plane, within a radius enclosing 100 MBH masses. We never find binary hosts to be prolate on any scale. The decaying MBHs slightly enhance the tangential anisotropy in the centre of the remnant due to angular momentum injection and the slingshot ejection of stars on nearly radial orbits. This latter effect results in about 1% of the remnant stars being expelled from the galactic nucleus. Finally, we do not find any strong connection between the remnant morphology and the binary hardening rate, which depends only on the inner density slope of the remnant galaxy. Our results suggest that MBH binaries are able to coalesce within a few Gyr, even if the binary is found to partially erase the merger-induced triaxiality from the remnant.