# Negative Dynamical Friction on compact objects moving through dense gas

## TL;DR

This paper proposes that powerful outflows from gravitating objects in dense gas can reverse the usual dynamical friction, leading to a negative force that accelerates the object instead of slowing it down.

## Contribution

It introduces the concept of negative dynamical friction caused by outflows disrupting the wake, a novel idea in the context of dense gas environments.

## Key findings

- Negative dynamical friction can occur with strong outflows.
- Outflows destroy the over-dense wake, creating an under-dense region.
- Potential implications for black hole dynamics in dense astrophysical environments.

## Abstract

An over-dense wake is created by a gravitating object moving through a gaseous medium, and this wake pulls back on the object and slows it down. This is conventional dynamical friction in a gaseous medium. We argue that if the object drives a sufficiently powerful outflow, the wake is destroyed and instead an extended under-dense region is created behind the object. In this case the overall gravitational force is applied in the direction of the object's motion, producing a negative dynamical friction (NDF). Black holes in dense gas drive powerful outflows and may experience the NDF, although extensive numerical work is probably needed to demonstrate or refute this conclusively. NDF may be important for stellar-mass black holes and neutron stars inside `common envelopes' in binary systems, for stellar mass black holes inside AGN discs, or for massive black holes growing through super-Eddington accretion in early Universe.