Si IV Column Densities Predicted from Non-Equilibrium Ionization Simulations of Turbulent Mixing Layers and High-Velocity Clouds

Abstract

We present the column densities of Si IV predicted from hydrodynamic simulations of turbulent mixing layers (TMLs) and high velocity clouds (HVCs). By using non-equilibrium ionization calculations, we traced the ionization states of silicon at each hydrodynamic time step in the simulations. The Si IV ions become abundant in the mixed gas that forms via mixing of cool cloud gas with hot ambient gas. Most of our TML models underpredict low-velocity Si IV column densities so that multiple layers are required to explain the observed column densities. However, the high-velocity Si IV column densities predicted from our HVC simulations overlap the observed ones. The ratios between the Si IV column densities presented here and the C IV and O VI column densities presented in Kwak & Shelton (2010) and Kwak et al. (2011) for the same hydrodynamic models are generally in better agreement with observations than are predictions from other models such as thermal conduction and shock heating. In time, the mixed material decelerates to the speed of the environmental gas and may contribute to the numbers of high ions observed at normal velocities in the Milky Way's halo. Following Henley et al. (2012), we estimate contributions to the observed normalvelocity high ions in the halo from four scenarios including decelerated HVC gas, extraplanar supernova remnants, galactic fountains, and photoionization by external radiation fields. We find that additional processes that produce more Si IV ions relative to other high ions are required in order to account for all of the observed normal-velocity Si IV ions in the halo.