Non-equilibrium Ionization States Within Galactic Outflows: Explaining Their O VI and N V Column Densities

Abstract: We present a suite of one-dimensional spherically-symmetric hydrodynamic simulations that study the atomic ionization structure of galactic outflows. We track the ionization state of the outflowing gas with a non-equilibrium atomic chemistry network that includes photoionization, photo-heating, and ion-by-ion cooling. Each simulation describes a steady-state outflow that is defined by its mass and energy input rates, sonic radius, metallicity, and UV flux from both the host galaxy and meta-galactic background. We find that for a large range of parameter choices, the ionization state of the material departs strongly from what it would be in photo-ionization equilibrium, in conflict with what is commonly assumed in the analysis of observations. In addition, nearly all the models reproduce the low N V to O VI column density ratios and the relatively high O VI column densities that are observed.