Si iv Column Densities Predicted from Non-Equilibrium Ionization
  Simulations of Turbulent Mixing Layers and High-Velocity Clouds
  We present predictions of the Si iv ions in turbulent mixing layers (TMLs)
between hot and cool gas and in cool high-velocity clouds (HVCs) that travel
through a hot halo, complementing the C iv, N v, and O vi predictions in Kwak &
Shelton, Kwak et al., and Henley et al. We find that the Si iv ions are most
abundant in regions where the hot and cool gases first begin to mix or where
the mixed gas has cooled significantly. The predicted column densities of high
velocity Si iv and the predicted ratios of Si iv to C iv and O vi found on
individual sightlines in our HVC simulations are in good agreement with
observations of high velocity gas. Low velocity Si iv is also seen in the
simulations, as a result of decelerated gas in the case of the HVC simulations
and when looking along directions that pass perpendicular to the direction of
motion in the TML simulations. The ratios of low velocity Si iv to C iv and O
vi in the TML simulations are in good agreement with those recorded for Milky
Way halo gas, while the ratio of Si iv to O vi from the decelerated gas in the
HVC simulations is lower than that observed at normal velocity in the Milky Way
halo. We attribute the shortfall of normal velocity Si iv to not having modeled
the effects of photoionization and, following Henley et al., consider a
composite model that includes decelerated HVC gas, supernova remnants, galactic
fountain gas, and the effect of photoionization.
