Hydrodynamic shielding in radiative multicloud outflows within multiphase galactic winds

ABSTRACT Stellar-driven galactic winds regulate the mass and energy content of star-forming galaxies. Emission- and absorption-line spectroscopy show 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.