The effect of rotation on the thermal instability of stratified galactic atmospheres - II. The formation of high-velocity clouds

Whether high-velocity clouds (HVCs) can form by condensation of the hot (T ∼ 10⁶ K) Galactic corona as a consequence of thermal instabilities has been controversial. Here, we re-examine this problem and we suggest that rotation of the corona might be a missing key ingredient. We do this by studying the evolution of the models of rotating galactic coronae presented in Sormani et al. (2018) under the presence of cooling and thermal conduction. We combine a linear stability analysis with the results of local and global hydrodynamical simulations. We find that condensations are likely to occur in regions where the corona has substantial rotational support. Under reasonably general assumptions on the rotation profile of the corona, the locations where condensations are expected are in remarkable agreement with the observed location of the major non-magellanic HVC complexes in our Galaxy (namely, at distances ≤ 15 kpc from the Sun and within 30° from the disc plane). We conclude that HVCs can form by thermal instabilities provided that (i) the corona is rotating substantially in the inner (R ≤ 50 kpc) parts, as suggested by current observational data and predicted by cosmological simulations of galaxy formation; and (ii) close to the disc the corona is well represented by a nearly equilibrium stratified rotating structure (as opposed to a fast-cooling flow). Our results also suggest that a better understanding of the disc-halo interface, including supernova feedback, is critical to understand the origin of HVCs.