Revisiting dynamical friction: the role of global modes and local wakes

TL;DR

This paper uses high-resolution N-body simulations to investigate the combined effects of local wakes and global modes on the orbital decay of satellites in galaxies, clarifying the physics behind dynamical friction.

Contribution

It provides a detailed analysis of how local and global perturbations jointly influence dynamical friction, advancing understanding beyond traditional local approaches.

Findings

Both local wakes and global modes significantly contribute to orbital decay.

Global modes can dominate the torque in certain orbital configurations.

High-resolution simulations effectively capture the interplay of local and global effects.

Abstract

The orbital decay of a perturber within a larger system plays a key role in the dynamics of many astrophysical systems -- from nuclear star clusters or globular clusters in galaxies, to massive black holes in galactic nuclei, to dwarf galaxy satellites within the dark matter halos of more massive galaxies. For many decades, there have been various attempts to determine the underlying physics and time-scales of the drag mechanism, ranging from the local dynamical friction approach to descriptions based on the back-reaction of global modes induced in the background system. We present ultra-high-resolution $N$-body simulations of massive satellites orbiting a Milky Way-like galaxy (with $> 10^8$ particles), that appear to capture both the local "wake" and the global "mode" induced in the primary halo. We address directly the mechanism of orbital decay from the combined action of local and global perturbations and specifically analyze where the bulk of the torque originates.