High-velocity OVI in and near the Milky Way

The Far Ultraviolet Spectroscopic Explorer (FUSE) has observed over three hundred fifty sight-lines to extragalactic targets. About one hundred fifty of these are of sufficient quality to measure O VI absorption in and near the Milky Way. High-velocity O VI absorption is detected in about 80% of the sight-lines, with the detection rate going up for sight-fines with higher signal-to-noise ratios. O VI is almost always seen in directions with previously known H I HVCs, including HVCs complex C, complex A, complex WD, the Magellanic Stream, and the Outer Arm. Studies of several sight-lines through complex C suggest that the O VI absorption is produced in conductive interfaces between the cool HVC and a hotter surrounding medium, most likely a corona around the Milky Way. The O VI detections associated with the Magellanic Stream imply that this hot corona has a radius of at least 50 kpc. About half of the detections of high-velocity O VI are in directions where no high-velocity H I was previously known. Some of these are probably associated with the Magellanic Stream, others may represent Local Group gas. Still others often show up as a distinct wing on the low-velocity absorption; these may either represent an outflow from the Milky Way associated with the Galactic Fountain, they may be extragalactic HVCs, or they may be tracing a wind from the Galactic Center. Distance limits to the high-velocity O VI are scarce, but at least one of the clouds appear to be more distant than 4 kpc.