Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity
  Clouds
  We revisit the reliability of metallicity estimates of high velocity clouds
with the help of hydrodynamical simulations. We quantify the effect of
accretion and viewing angle on metallicity estimates derived from absorption
lines. Model parameters are chosen to provide strong lower limits on cloud
contamination by ambient gas. Consistent with previous results, a cloud
traveling through a stratified halo is contaminated by ambient material to the
point that <10% of its mass in neutral hydrogen consists of original cloud
material. Contamination progresses nearly linearly with time, and it increases
from head to tail. Therefore, metallicity estimates will depend on the
evolutionary state of the cloud, and on position. While metallicities change
with time by more than a factor of 10, well beyond observational uncertainties,
most lines-of-sight range only within those uncertainties at any given time
over all positions. Metallicity estimates vary with the cloud's inclination
angle within observational uncertainties. The cloud survives the infall through
the halo because ambient gas continuously condenses and cools in the cloud's
wake and thus appears in the neutral phase. Therefore, the cloud observed at
any fixed time is not a well-defined structure across time, since material gets
constantly replaced. The thermal phases of the cloud are largely determined by
the ambient pressure. Internal cloud dynamics evolve from drag gradients caused
by shear instabilities, to complex patterns due to ram-pressure shielding,
leading to a peloton effect, in which initially lagging gas can catch up to and
even overtake the head of the cloud.
