How cold gas continuously entrains mass and momentum from a hot wind
  The existence of fast moving, cold gas ubiquitously observed in galactic
winds is theoretically puzzling, since the destruction time of cold gas is much
smaller than its acceleration time. In previous work, we showed that cold gas
can accelerate to wind speeds and grow in mass if the radiative cooling time of
mixed gas is shorter than the cloud destruction time. Here, we study this
process in much more detail, and find remarkably robust cloud acceleration and
growth in a wide variety of scenarios. Radiative cooling, rather than the
Kelvin-Helmholtz instability, enables self-sustaining entrainment of hot gas
onto the cloud via cooling-induced pressure gradients. Indeed, growth peaks
when the cloud is almost co-moving. The entrainment velocity is of order the
cold gas sound speed, and growth is accompanied by cloud pulsations. Growth is
also robust to the background wind and initial cloud geometry. In an adiabatic
Chevalier-Clegg type wind, for instance, the mass growth rate is constant.
Although growth rates are similar with magnetic fields, cloud morphology
changes dramatically, with low density, magnetically supported filaments which
have a small mass fraction but dominate by volume. This could bias absorption
line observations. Cloud growth from entraining and cooling hot gas can
potentially account for the cold gas content of the CGM. It can also fuel star
formation in the disk as cold gas recycled in a galactic fountain accretes and
cools halo gas. We speculate that galaxy-scale simulations should converge in
cold gas mass once cloud column densities of ${\rm N} \sim 10^{18} \ {\rm
cm^{-2}}$ are resolved.
