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We present hydrodynamic simulations of high-velocity clouds (HVCs) travelingthrough the hot, tenuous medium in the Galactic halo. A suite of models wascreated using the FLASH hydrodynamics code, sampling various cloud sizes,densities, and velocities. In all cases, the cloud-halo interaction ablatesmaterial from the clouds. The ablated material falls behind the clouds, whereit mixes with the ambient medium to produce intermediate-temperature gas, someof which radiatively cools to less than 10,000 K. Using a non-equilibriumionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen,and oxygen in the gas throughout the simulation period. We presentobservation-related predictions, including the expected H I and high ion (C IV,N V, and O VI) column densities on sight lines through the clouds as functionsof evolutionary time and off-center distance. The predicted column densitiesoverlap those observed for Complex C. The observations are best matched byclouds that have interacted with the Galactic environment for tens to hundredsof megayears. Given the large distances across which the clouds would travelduring such time, our results are consistent with Complex C having anextragalactic origin. The destruction of HVCs is also of interest; the smallestcloud (initial mass \approx 120 Msun) lost most of its mass during thesimulation period (60 Myr), while the largest cloud (initial mass \approx 4e5Msun) remained largely intact, although deformed, during its simulation period(240 Myr).

Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions