Do high-velocity clouds form by thermal instability?

TL;DR

This paper investigates whether high-velocity clouds around galaxies can form through thermal instability in hot galactic coronae, concluding that this process is unlikely under typical conditions.

Contribution

It demonstrates that thermal instability is an improbable mechanism for high-velocity cloud formation in galactic coronae unless specific idealized conditions are met.

Findings

Thermal instability requires nearly flat entropy profiles to form clouds.

Cold clouds smaller than 10 kpc can only form beyond 100 kpc in the Milky Way.

Cloud formation in inner halos is limited to low-mass systems.

Abstract

We examine the proposal that the HI "high-velocity" clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than 10 kpc could form in the corona of the Milky Way only at radii larger than 100 kpc, in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies.