The physics of multiphase gas flows: fragmentation of a radiatively
  cooling gas cloud in a hot wind
  Galactic winds exhibit a multiphase structure that consists of hot-diffuse
and cold-dense phases. Here we present high-resolution idealised simulations of
the interaction of a hot supersonic wind with a cold cloud with the moving-mesh
code arepo in setups with and without radiative cooling. We demonstrate that
cooling causes clouds with sizes larger than the cooling length to fragment in
two- and three-dimensional simulations (2D and 3D). We confirm earlier 2D
simulations by McCourt et al. 2018 and highlight differences of the shattering
processes of 3D clouds that are exposed to a hot wind. The fragmentation
process is quantified with a friends-of-friends analysis of shattered cloudlets
and density power spectra. Those show that radiative cooling causes the power
spectral index to gradually increase when the initial cloud radius is larger
than the cooling length and with increasing time until the cloud is fully
dissolved in the hot wind. A resolution of around 1 pc is required to reveal
the effect of cooling-induced fragmentation of a 100 pc outflowing cloud. Thus,
state-of-the-art cosmological zoom simulations of the circumgalactic medium
(CGM) fall short by orders of magnitudes from resolving this fragmentation
process. This physics is, however, necessary to reliably model observed column
densities and covering fractions of Lyman-$\alpha$ haloes, high-velocity
clouds, and broad-line regions of active galactic nuclei.
