Dynamical Friction of Double Perturbers in a Gaseous Medium

TL;DR

This paper investigates how the combined gravitational wakes of two orbiting objects in a gaseous medium affect their orbital decay, revealing that mutual interactions can slow down the process, especially in subsonic regimes.

Contribution

It introduces a semi-analytic model for the dynamical friction on double perturbers in a gaseous medium, highlighting the impact of their mutual wakes on orbital decay.

Findings

The wake of each perturber becomes asymmetric, forming an overdense tail.

The tail exerts a positive torque, partially counteracting the negative drag.

Orbital decay can be significantly delayed in the presence of a companion, especially subsonically.

Abstract

In many astrophysical situations, as in the coalescence of supermassive black hole pairs at gas rich galactic nuclei, the dynamical friction experienced by an object is a combination of its own wake as well as the wakes of its companions. Using a semi-analytic approach, we investigate the composite wake due to, and the resulting drag forces on, double perturbers that are placed at the opposite sides of the orbital center and move on a circular orbit in a uniform gaseous medium. The circular orbit makes the wake of each perturber asymmetric, creating an overdense tail at the trailing side. The tail not only drags the perturber backward but it also exerts a positive torque on the companion. For equal-mass perturbers, the positive torque created by the companion wake is, on average, a fraction ~40-50% of the negative torque created by its own wake, but this fraction may be even larger for perturbers moving subsonically. This suggests that the orbital decay of a perturber in a double system, especially in the subsonic regime, can take considerably longer than in isolation. We provide the fitting formulae for the forces due to the companion wake and discuss our results in light of recent numerical simulations for mergers of binary black holes.