O VI, N V, and C IV in the Galactic Halo. II. Velocity-Resolved Observations with the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer

摘要

We present a survey of N V and O VI (and, where available, C IV) in the Galactic halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST) along 34 sight lines. These ions are usually produced in nonequilibrium processes such as shocks, evaporative interfaces, or rapidly cooling gas, and thus trace the dynamics of the interstellar medium. Searching for global trends in integrated and velocity-resolved column density ratios, we find large variations in most measures, with some evidence for a systematic trend of higher ionization (lower N V/O VI column density ratio) at larger positive line-of-sight velocities. The slopes of log [N(N V)/N(O VI)] per unit velocity range from -0.015 to +0.005, with a mean of -0.0032 ± 0.0022(r) ± 0.0014(sys) dex?(km?s-1)-1. We compare this data set with models of velocity-resolved high-ion signatures of several common physical structures. The dispersion of the ratios, O VI/N V/C IV, supports the growing belief that no single model can account for hot halo gas, and in fact some models predict much stronger trends than are observed. It is important to understand the signatures of different physical structures to interpret specific lines of sight and future global surveys.