Dynamics of intermediate mass black holes in globular clusters. Wander radius and anisotropy profiles

TL;DR

This study introduces a new efficient simulation method for globular clusters with intermediate mass black holes, revealing their impact on core collapse, velocity dispersion, and IMBH wander behavior, with implications for observational detection.

Contribution

The paper presents a novel Multi-Particle Collision simulation approach enabling large-scale globular cluster modeling with IMBHs, overcoming previous computational limitations and exploring new dynamical behaviors.

Findings

IMBH presence accelerates core collapse and results in shallower cores.

IMBHs lower central velocity dispersion regardless of mass function.

IMBH wander radius distribution is leptokurtic, affecting off-center IMBH detection.

Abstract

We recently introduced a new method for simulating collisional gravitational N-body systems with approximately linear time scaling with $N$, based on the Multi-Particle Collision (MPC) scheme, previously applied in Plasma Physics. We simulate globular clusters with a realistic number of stellar particles (at least up to several times $10^6$) on a standard workstation. We simulate clusters hosting an intermediate mass black hole (IMBH), probing a broad range of BH-cluster and BH-average-star mass ratios, unrestricted by the computational constraints affecting direct N-body codes. We use either single mass models or models with a Salpeter mass function, with the IMBH initially sitting at the centre. The force exerted by and on the IMBH is evaluated with a direct scheme. We measure the evolution of the Lagrangian radii and core density and velocity dispersion over time. In addition, we study the evolution of the velocity anisotropy profiles. We find that models with an IMBH undergo core collapse at earlier times, the larger the IMBH mass the shallower, with an approximately constant central density at core collapse. The presence of an IMBH tends to lower the central velocity dispersion. These results hold independently of the mass function. For the models with Salpeter MF we observe that equipartition of kinetic energies is never achieved. Orbital anisotropy at large radii appears driven by energetic escapers on radial orbits. We measure the wander radius. Among the results we obtained, which mostly confirm or extend previously known trends that had been established over the range of parameters accessible to direct N-body simulations, we underline that the leptokurtic nature of the IMBH wander radius distribution might lead to IMBHs presenting as off-center more frequently than expected, with implications on observational IMBH detection.