Interaction of a cold cloud with a hot wind: the regimes of cloud growth
  and destruction and the impact of magnetic fields
  Multiphase galaxy winds, the accretion of cold gas through galaxy haloes, and
gas stripping from jellyfish galaxies are examples of interactions between cold
and hot gaseous phases. There are two important regimes in such systems. A
sufficiently small cold cloud is destroyed by the hot wind as a result of
Kelvin-Helmholtz instabilities, which shatter the cloud into small pieces that
eventually mix and dissolve in the hot wind. On the contrary, stripped cold gas
from a large cloud mixes with the hot wind to intermediate temperatures, and
then becomes thermally unstable and cools, causing a net accretion of hot gas
to the cold tail. Using the magneto-hydrodynamical code AREPO, we perform cloud
crushing simulations and test analytical criteria for the transition between
the growth and destruction regimes to clarify a current debate in the
literature. We find that the hot-wind cooling time sets the transition radius
and not the cooling time of the mixed phase. Magnetic fields modify the
wind-cloud interaction. Draping of wind magnetic field enhances the field
upstream of the cloud and fluid instabilities are suppressed by a turbulently
magnetised wind beyond what is seen for a wind with a uniform magnetic field.
We furthermore predict jellyfish galaxies to have ordered magnetic fields
aligned with their tails. We finally discuss how the results of idealised
simulations can be used to provide input to subgrid models in cosmological
(magneto-)hydrodynamical simulations, which cannot resolve the detailed
small-scale structure of cold gas clouds in the circum-galactic medium.
