The Evolution of Kicked Stellar-Mass Black Holes in Star Cluster Environments

TL;DR

This paper investigates how dynamical friction models predict the orbital decay of kicked stellar-mass black holes in star clusters, revealing limitations of classical models and proposing a new linear decay formalism.

Contribution

It introduces a new linear decay model for black hole orbital evolution that accounts for high kick velocities and local potential variations, improving upon classical dynamical friction theories.

Findings

Classical dynamical friction models often underestimate energy loss for high kick velocities.

Two-body encounters can cause energy gains, deviating from standard theory.

Orbital decay transitions from exponential to linear at large kick velocities.

Abstract

We consider how dynamical friction acts on black holes that receive a velocity kick while located at the center of a gravitational potential, analogous to a star cluster, due to either a natal kick or the anisotropic emission of gravitational waves during a black hole-black hole merger. Our investigation specifically focuses on how well various Chandrasekhar-based dynamical friction models can predict the orbital decay of kicked black holes with $m_{bh} \lesssim 100 M_\odot$ due to an inhomogeneous background stellar field. In general, the orbital evolution of a kicked black hole follows that of a damped oscillator where two-body encounters and dynamical friction serve as sources of damping. However, we find models for approximating the effects of dynamical friction do not accurately predict the amount of energy lost by the black hole if the initial kick velocity $v_{k}$ is greater than the stellar velocity dispersion $\sigma$. For all kick velocities, we also find that two-body encounters with nearby stars can cause the energy evolution of a kicked BH to stray significantly from standard dynamical friction theory as encounters can sometimes lead to an energy gain. For larger kick velocities, we find the orbital decay of a black hole departs from classical theory completely as the black hole's orbital amplitude decays linearly with time as opposed to exponentially. Therefore, we have developed a linear decay formalism which scales linearly with black hole mass and $\frac{v_{k}}{\sigma}$ in order to account for the variations in the local gravitational potential.