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K. R. Sembach (Johns Hopkins University), B. D. Savage (University of Wisconsin), J. M. Shull (University of Colorado), E. B. Jenkins (Princeton University), E. M. Murphy (Johns Hopkins University), D. G. York (University of Chicago), FUSE Science Team
We have used moderate-resolution (FWHM ~ 25 km/s) spectra of AGNs and QSOs observed by the Far Ultraviolet Spectroscopic Explorer to make measurements of absorption by hot gas in high velocity clouds (HVCs). Seven of the 11 sight lines studied exhibit high velocity (V(LSR) > 100 km/sec) O VI 1031.9 absorption with log N(O VI) = 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 about 30 percent, with a full range of 10-70 percent. The O VI detections imply that hot (log T(K) = 5-6), 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. 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.
This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support has been provided by NASA contract NAS5-32985.
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