Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity
  High Ions
  We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling
through the hot, tenuous medium in the Galactic halo. A suite of models was
created using the FLASH hydrodynamics code, sampling various cloud sizes,
densities, and velocities. In all cases, the cloud-halo interaction ablates
material from the clouds. The ablated material falls behind the clouds, where
it mixes with the ambient medium to produce intermediate-temperature gas, some
of which radiatively cools to less than 10,000 K. Using a non-equilibrium
ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen,
and oxygen in the gas throughout the simulation period. We present
observation-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 functions
of evolutionary time and off-center distance. The predicted column densities
overlap those observed for Complex C. The observations are best matched by
clouds that have interacted with the Galactic environment for tens to hundreds
of megayears. Given the large distances across which the clouds would travel
during such time, our results are consistent with Complex C having an
extragalactic origin. The destruction of HVCs is also of interest; the smallest
cloud (initial mass \approx 120 Msun) lost most of its mass during the
simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5
Msun) remained largely intact, although deformed, during its simulation period
(240 Myr).
