How do cold gas clouds survive inside hot galactic halos?
In plain words
Halos contain cool clouds at about ten thousand degrees moving through gas a hundred times hotter. Such clouds should be shredded and evaporated quickly, yet they are common.
Precise statement
Clouds at $T \sim 1e4\ \mathrm{K}$ moving through halo gas at $T \sim 1e6\ \mathrm{K}$ should be destroyed by Kelvin-Helmholtz instabilities in a few cloud-crushing times $t_{\mathrm{cc}}$. Simulations find clouds grow when the mixing-layer cooling time is short compared with $t_{\mathrm{cc}}$ (Gronke and Oh 2018). Determine whether this criterion, modified by magnetic fields, thermal conduction and cosmic rays, predicts the observed cloud sizes, velocities and covering fractions.
What would settle it
Converged radiative MHD simulations with conduction and cosmic rays that match absorption-line cloud sizes and kinematics, tested against small-scale structure seen in lensed quasar sightlines.
Status in the literature
A survival criterion for idealized clouds is established in simulations; its validity with full halo physics and its observational test remain open.