Entrainment in Trouble: Cool Cloud Acceleration and Destruction in Hot
  Supernova-Driven Galactic Winds
  Efficient thermalization of overlapping supernovae within star-forming
galaxies may produce a supernova-heated fluid that drives galactic winds. For
fiducial assumptions about the timescale for cloud shredding from
high-resolution simulations (which neglect magnetic fields) we show that cool
clouds with temperature from $T_{c}\sim 10^{2}-10^{4}$ K seen in emission and
absorption in galactic winds cannot be accelerated to observed velocities by
the ram pressure of a hot wind. Taking into account both the radial structure
of the hot flow and gravity, we show that this conclusion holds over a wide
range of galaxy, cloud, and hot wind properties. This finding calls into
question the prevailing picture whereby the cool atomic gas seen in galactic
winds is entrained and accelerated by the hot flow. Given these difficulties
with ram pressure acceleration, we discuss alternative models for the origin of
high velocity cool gas outflows. Another possibility is that magnetic fields in
cool clouds are sufficiently important that they prolong the cloud's life. For
$T_{c}=10^{3}$\,K and $10^{4}$\,K clouds, we show that if conductive
evaporation can be neglected, the cloud shredding timescale must be $\sim15$
and 5 times longer, respectively, than the values from hydrodynamical
simulations in order for cool cloud velocities to reach those seen in
observations.
