On the Survival of Cool Clouds in the Circum-Galactic Medium
  We explore the survival of cool clouds in multi-phase circum-galactic media.
We revisit the "cloud crushing problem" in a large survey of simulations
including radiative cooling, self-shielding, self-gravity, magnetic fields, and
anisotropic Braginskii conduction and viscosity (with saturation). We explore a
wide range of parameters including cloud size, velocity, ambient temperature
and density, as well as a variety of magnetic field configurations and cloud
turbulence. We find that realistic magnetic fields and turbulence have weaker
effects on cloud survival; the most important physics is radiative cooling and
conduction. Self-gravity and self-shielding are important for clouds which are
initially Jeans-unstable, but largely irrelevant otherwise.
Non-self-gravitating, realistically magnetized clouds separate into four
regimes: (1) At low column densities, clouds evaporate rapidly via conduction.
(2) A "failed pressure confinement" regime, where the ambient hot gas cools too
rapidly to provide pressure confinement for the cloud. (3) An "infinitely
long-lived" regime, in which the cloud lifetime becomes longer than the cooling
time of gas swept up in the leading bow shock, so the cloud begins to accrete
and grow. (4) A "classical cloud destruction" regime, where clouds are
eventually destroyed by instabilities. In the final regime, the cloud lifetime
can exceed the naive cloud-crushing time owing to conduction-induced
compression. However, small and/or slow-moving clouds can also evaporate more
rapidly than the cloud-crushing time. We develop simple analytic models that
explain the simulated cloud destruction times in this regime.
