Thermal instability in rotating galactic coronae
  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.
