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

TL;DR

This study uses non-equilibrium ionization simulations to predict Si iv ion distributions in turbulent mixing layers and high-velocity clouds, aligning well with observations and highlighting the importance of mixing and cooling processes.

Contribution

It provides new predictions of Si iv in turbulent mixing layers and high-velocity clouds, incorporating non-equilibrium ionization effects and comparing results with observational data.

Findings

Si iv is most abundant during initial mixing and cooling phases.

Predicted Si iv column densities match observed high velocity gas.

Ratios of Si iv to C iv and O vi agree with Milky Way halo observations.

Abstract

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.