Demystifying Strange O <scp>vi</scp> Line Widths with Hydrodynamic Simulations

Abstract We present a survey of O vi line widths obtained from hydrodynamic simulations that model the mixing between high-velocity clouds and the circumgalactic medium and track the nonequilibrium ionization populations of the ions. We run 10 simulations with various physical conditions of the clouds and ambient environments, so that our results can be compared to observations of various cloud environments. Synthetic spectra are created for the simulated sight lines that contain O vi . The range of our Doppler broadening parameter, b , is from ∼6 to ∼107 km s −1 . We calculate the thermal and nonthermal contributions to the b values. Both b and the thermal contribution to b can be substantially less than the collisional ionizational equilibrium (CIE) value. These narrow line widths are due to the time delay in recombination from O vi to O v in mixing regions, which results in substantial amounts of O vi at temperatures below the 2.9 × 10 5 K CIE temperature. Our results show that nonequilibrium collisional ionization and mixing can produce the narrow line widths that are seen in multiple observations.