Far Ultraviolet Spectroscopic Explorer observations of O vi absorption in the galactic halo

Far-ultraviolet spectra of 11 active galactic nuclei observed by Far Ultrav iolet Spectroscopic Explorer (FUSE) are analyzed to obtain measures of O VI lambda 1031.93 absorption occurring over very long paths through Milky Way halo gas. Strong O VI absorption is detected along 10 of 11 sight lines. V alues of log [N(O VI) sin \b\] range from 13.80 to 14.64, with a median val ue of 14.21. The observations reveal the existence of a widespread but irre gular distribution of O VI in the Milky Way halo. Combined with estimates o f the O VI midplane density, n(0) = 2 x 10(-8) cm(-3), from the Copernicus satellite, the FUSE observations imply an O VI exponential scale height of 2.7 +/- 0.4 kpc. We find that N(C IV)/N(O VI) ranges from similar to 0.15 i n the disk to similar to 0.6 along four extragalactic sight lines. The chan ging ionization state of the gas from the disk to the halo is consistent wi th a systematic decrease in the scale heights of Si IV, C IV, N V, to O VI from similar to 5.1 to similar to 2.7 kpc. While conductive heating models can account for the highly ionized atoms at low \z\,a combination of models land processes) appears to be required to explain the highly ionized atoms found in the halo. The greater scale heights of Si IV and C IV compared to O VI suggests that some of the Si IV and C IV in the halo is produced in t urbulent mixing layers or by photoionization by hut halo stars or the extra galactic background.