Growth and structure of multiphase gas in the cloud-crushing problem
  with cooling
  We revisit the problem of the growth of dense/cold gas in the cloud-crushing
setup with radiative cooling. The relative motion between the dense cloud and
the diffuse medium produces a turbulent boundary layer of mixed gas with a
short cooling time. This mixed gas may explain the ubiquity of the range of
absorption/emission lines observed in various sources such as the
circumgalactic medium and galactic/stellar/AGN outflows. Recently Gronke & Oh
showed that the efficient radiative cooling of the mixed gas can lead to the
continuous growth of the dense cloud. They presented a threshold cloud size for
the growth of dense gas which was contradicted by the more recent works of Li
et al. & Sparre et al. These thresholds are qualitatively different as the
former is based on the cooling time of the mixed gas whereas the latter is
based on the cooling time of the hot gas. Our simulations agree with the
threshold based on the cooling time of the mixed gas. We argue that the
radiative cloud-crushing simulations should be run long enough to allow for the
late-time growth of the dense gas due to cooling of the mixed gas but not so
long that the background gas cools catastrophically. Moreover, the simulation
domain should be large enough that the mixed gas is not lost through the
boundaries. While the mixing layer is roughly isobaric, the emissivity of the
gas at different temperatures is fundamentally different from an isobaric
single-phase steady cooling flow.
