Nonequilibrium Ionization States within Galactic Outflows: Explaining Their O vi and N v Column Densities

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 nonequilibrium atomic chemistry network that includes photoionization, photoheating, 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 metagalactic 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 photoionization 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.