[ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of O [CSC]vi[/CSC] in High-Velocity Clouds

Abstract

We have used moderate-resolution (FWHM ≈ 25 km s-1) spectra of active galactic nuclei and QSOs observed by the Far Ultraviolet Spectroscopic Explorer to make the first definitive measurements of absorption by hot gas in high-velocity clouds (HVCs) at large distances from the Galactic plane. Seven of the 11 sight lines studied exhibit high-velocity ( > 100 km s-1) O VI λ1031.93 absorption with log N(O ) ≈ 13.79-14.62. High-velocity O VI absorption is detected in the distant gas of H I HVC complex C, the Magellanic Stream, several HVCs believed to be in the Local Group, and the outer Galaxy. The fraction of O VI in HVCs along the seven sight lines containing high-velocity O VI averages ~30%, with a full range of ~10%-70%. The O VI detections imply that hot (T ~ 3 × 105 K), collisionally ionized gas is an important constituent of the HVCs since O VI is difficult to produce by photoionization unless the path lengths over which the absorption occurs are very large (>100 kpc). The association of O VI with H I HVCs in many cases suggests that the O VI may be produced at interfaces or mixing layers between the H I clouds and hot, low-density gas in the Galactic corona or Local Group. Alternatively, the O VI may originate within cooling regions of hot gas clouds as they are accreted onto the Galaxy.