Prograde and retrograde stars in nuclear cluster mergers. Evolution of the supermassive black hole binary and the host galactic nucleus

TL;DR

This study uses N-body simulations to analyze how prograde and retrograde stars in merging nuclear star clusters influence the evolution of supermassive black hole binaries and the surrounding stellar dynamics, impacting gravitational wave event rates.

Contribution

It provides detailed insights into the orbital distributions and kinematic structures of stars post-merger, highlighting their effects on SMBH binary evolution and EMRI production.

Findings

Prograde stars form a flattened structure, retrograde stars are more spherical.

The stellar orbital distribution depends on SMBH mass ratio and merger orbit.

Prograde stars are closer to SMBH binaries, affecting EMRI formation.

Abstract

We address the orbital distribution of stars in merging nuclear star clusters (NSCs) and the subsequent effects on supermassive black hole binary (SMBHB) evolution. We ran direct-summation $N$-body simulations with different initial conditions to do a detailed study of the resulting NSC after their progenitors had merged. Our findings reveal that prograde stars form a flattened structure, while retrograde stars have a more spherical distribution. The axial ratios of the prograde component vary based on the presence and mass ratio of the SMBHs. The fraction of prograde and retrograde stars depends on the merger orbital properties and the SMBH mass ratio. The interactions of retrograde stars with the SMBHB affect the eccentricity and separation evolution of the binary. Our analysis reveals a strong correlation between the angular momentum and eccentricity of the SMBH binary. This relationship could serve as a means to infer information about the stellar dynamics surrounding the binary. We find that prograde orbits are particularly close to the binary of SMBHs, a promising fact regarding extreme mass ratio inspiral (EMRI) production. Moreover, prograde and retrograde stars have different kinematic structures, with the prograde stars typically rotating faster than the retrograde ones. The line-of-sight velocity and velocity dispersion, as well as the velocity anisotropy of each NSC, depend on the initial merger orbital properties and SMBH mass ratios. The prograde and retrograde stars always show different behaviours. The distribution of stellar orbits and the dynamical properties of each kinematic population can potentially be used as a way to tell the properties of the parent nuclei apart, and has an important impact on expected rates of EMRIs, which will be detected by future gravitational wave observatories such as the Laser Interferometer Space Antenna (LISA). [abridged]