Far Ultraviolet Spectroscopic Explorer observations of O vi in high-velocity clouds

We have used moderate-resolution (FWHM approximate to 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 Il sight lines studied exhibit high-velocity (/V-LSR/ > 100 km s(-1)) O VI lambda 1031.93 absorption with log N(O vr) approximate to 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 similar to 30%, with a full range of similar to 10%-70%. The O vr detections imply that hut (T similar to 3 x 10(5) 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 vr may originate within cooling regions of hot gas clouds as they are accreted onto the Galaxy.