Ultraviolet Line Intensities of Intermediate and High Ions Predicted from Non-equilibrium Ionization Simulations

Abstract

We predict the UV emission line intensities produced by turbulent mixing layers (TMLs) between hot and warm gas and in high-velocity clouds (HVCs) traveling through hot gas such as that in the extended Galactic halo, circumgalactic medium, and intergalactic space. The calculations include the effects of hydrodynamic mixing, cooling, and non-equilibrium ionization and were done with the FLASH code and the Raymond and Smith code. A variety of mixing-layer and cloud conditions were simulated, resulting in 13 individual models. The intensities of 21 UV emission lines or doublets of 14 ions of carbon, nitrogen, oxygen, and silicon in TMLs and HVCs are tabulated. These ions are C ii, C iii, C iv, N ii, N iii, N iv, N v, O iii, O iv, O v, O vi, Si ii, Si iii, and Si iv. Owing to the relatively small assumed pressures and depths, our predicted intensities are small. However, when properly scaled to account for the pressures and depths of such gas in other places, the predicted intensities can be used to estimate the sensitivity needed for observations of the diffuse UV emission or compared with observations of a variety of astrophysical structures. The ratios of the predicted Si iv, C iv, N v, and O vi intensities are expected to remain invariant when the predictions are scaled to correspond to denser regions. The ratios of the predicted Si iv, C iv, and O vi intensities are compared with those of regions in our Galaxy that were observed by SPEAR/FIMS and the Far Ultraviolet Spectroscopic Explorer and with those predicted for other models, including an analytical model of TMLs and a model with a magnetized thermal conduction front.