2017ApJ...842..102G

High-velocity clouds (HVCs) hydrodynamically interact with their surroundings. In scenarios with small clouds, the net result is erosion of the HVC, but in scenarios with large, massive clouds, it is capture and cooling of environmental gas by the cloud. In order to examine these effects over long periods of time, we made detailed three-dimensional hydrodynamic FLASH simulations of massive HVCs (1.35 x 10⁵ M_☉ to 1.35 x 10⁸ M_☉) traveling through hot, low-density media like that in the extended Galactic halo, circumgalactic media, and intergalactic space for at least 200 Myr. By setting the metallicity of the clouds to differ from that of the ambient gas, we were able to track the transfer of material between the two media. We found that massive clouds condense substantial amounts of ambient gas, up to 100% of their initial mass in certain cases. This gas cools to temperatures below 1.0 x 10⁴ K, but retains some high ions. The amount of condensed gas depends on the cloud mass, the ambient density, and the cloud density and temperature, but apparently not on the velocity. We discuss the ramifications for cloud survivability and for the transport of halo, circumgalactic, and intergalactic gas to the disk of the Galaxy aboard massive HVCs.