Growth and structure of multiphase gas in the cloud-crushing problem with cooling. (arXiv:2009.00525v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kanjilal_V/0/1/0/all/0/1">Vijit Kanjilal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dutta_A/0/1/0/all/0/1">Alankar Dutta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sharma_P/0/1/0/all/0/1">Prateek Sharma</a>
We revisit the problem of the growth of dense/cold gas in the cloud-crushing
setup with radiative cooling. This model problem captures the interaction of a
pre-existing cold cloud with a hot and dilute background medium, through which
it moves. The relative motion produces a turbulent boundary layer of mixed gas
with a short cooling time. The cooling of this mixed gas in the wake of clouds
may explain the ubiquity of a multiphase gas in various sources such as the
circumgalactic medium (CGM) and galactic/stellar/AGN outflows. In absence of
radiative cooling, the cold gas is mixed in the hot medium before it becomes
comoving with it. Recently Gronke & Oh, based on 3D hydrodynamic simulations,
showed that with efficient radiative cooling of the mixed gas in the turbulent
boundary layer, cold clouds can continuously grow and entrain mass from the
diffuse background. They presented an analytic criterion for such growth to
happen — namely, the cooling time of the mixed phase be shorter than the
classic cloud-crushing time ($t_{rm cc}=chi^{1/2}R_{rm cl}/v_{rm wind}$).
Another recent work (Li et al.) contradicted this criterion and presented a
threshold based on the properties of the hot wind. In this work we carry out
extensive 3D hydrodynamics simulations of cloud-crushing with radiative cooling
and find results consistent with Gronke & Oh. Li et al. see cloud destruction
because of the combination of a small box-size and a high resolution, and their
definition of cloud destruction. The apparent discrepancy between the results
of the various groups can be resolved if we recognize that a high density
contrast ($chi$) and Mach number imply a longer cooling time of the hot phase,
and such simulations must be run for longer to test for the possibility of cold
mass growth in the radiative mixing layer. [abridged]
We revisit the problem of the growth of dense/cold gas in the cloud-crushing
setup with radiative cooling. This model problem captures the interaction of a
pre-existing cold cloud with a hot and dilute background medium, through which
it moves. The relative motion produces a turbulent boundary layer of mixed gas
with a short cooling time. The cooling of this mixed gas in the wake of clouds
may explain the ubiquity of a multiphase gas in various sources such as the
circumgalactic medium (CGM) and galactic/stellar/AGN outflows. In absence of
radiative cooling, the cold gas is mixed in the hot medium before it becomes
comoving with it. Recently Gronke & Oh, based on 3D hydrodynamic simulations,
showed that with efficient radiative cooling of the mixed gas in the turbulent
boundary layer, cold clouds can continuously grow and entrain mass from the
diffuse background. They presented an analytic criterion for such growth to
happen — namely, the cooling time of the mixed phase be shorter than the
classic cloud-crushing time ($t_{rm cc}=chi^{1/2}R_{rm cl}/v_{rm wind}$).
Another recent work (Li et al.) contradicted this criterion and presented a
threshold based on the properties of the hot wind. In this work we carry out
extensive 3D hydrodynamics simulations of cloud-crushing with radiative cooling
and find results consistent with Gronke & Oh. Li et al. see cloud destruction
because of the combination of a small box-size and a high resolution, and their
definition of cloud destruction. The apparent discrepancy between the results
of the various groups can be resolved if we recognize that a high density
contrast ($chi$) and Mach number imply a longer cooling time of the hot phase,
and such simulations must be run for longer to test for the possibility of cold
mass growth in the radiative mixing layer. [abridged]
http://arxiv.org/icons/sfx.gif
