Thermal instability in rotating galactic coronae

TL;DR

This paper analyzes the thermal stability of rotating galactic coronae, revealing conditions under which perturbations grow or are damped, and suggesting that cool overdensities unlikely form internally in galaxy atmospheres.

Contribution

It provides a linear-perturbation analysis of thermal stability in rotating, stratified galactic coronae, highlighting the stabilizing effects of thermal conduction and differential rotation.

Findings

Axisymmetric perturbations grow unless damped by thermal conduction.

Non-axisymmetric perturbations are stabilized by thermal conduction with differential rotation.

Cool overdensities are unlikely to form internally, implying an external origin for high-velocity clouds.

Abstract

The thermal stability of rotating, stratified, unmagnetized atmospheres is studied by means of linear-perturbation analysis, finding stability, overstability or instability, depending on the properties of the gas distribution, but also on the nature of the perturbations. In the relevant case of distributions with outward-increasing specific entropy and angular momentum, axisymmetric perturbations grow exponentially, unless their wavelength is short enough that they are damped by thermal conduction; non-axisymmetric perturbations typically undergo overstable oscillations in the limit of zero conductivity, but are effectively stabilized by thermal conduction, provided rotation is differential. To the extent that the studied models are representative of the poorly constrained hot atmospheres of disc galaxies, these results imply that blob-like, cool overdensities are unlikely to grow in galactic coronae, suggesting an external origin for the high-velocity clouds of the Milky Way.