Hydrodynamic shielding in radiative multicloud outflows within multiphase galactic winds
arxiv(2024)
Abstract
Stellar-driven galactic winds regulate the mass and energy content of
star-forming galaxies. Emission- and absorption-line spectroscopy shows that
these outflows are multiphase and comprised of dense gas clouds embedded in
much hotter winds. Explaining the presence of cold gas in such environments is
a challenging endeavour that requires numerical modelling. In this paper we
report a set of 3D hydrodynamical simulations of supersonic winds interacting
with radiative and adiabatic multicloud systems, in which clouds are placed
along a stream and separated by different distances. As a complement to
previous adiabatic, subsonic studies, we demonstrate that hydrodynamic
shielding is also triggered in supersonic winds and operates differently in
adiabatic and radiative regimes. We find that the condensation of warm, mixed
gas in between clouds facilitates hydrodynamic shielding by replenishing dense
gas along the stream, provided that its cooling length is shorter than the
cloud radius. Small separation distances between clouds also favour
hydrodynamic shielding by reducing drag forces and the extent of the mixing
region around the clouds. In contrast, large separation distances promote
mixing and dense gas destruction via dynamical instabilities. The transition
between shielding and no-shielding scenarios across different cloud separation
distances is smooth in radiative supersonic models, as opposed to their
adiabatic counterparts for which clouds need to be in close proximity. Overall,
hydrodynamic shielding and re-condensation are effective mechanisms for
preserving cold gas in multiphase flows for several cloud-crushing times, and
thus can help understand cold gas survival in galactic winds.
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