Distribution and kinematics of O VI in the Galactic halo

Far-Ultraviolet Spectroscopic Explorer ( FUSE) spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI lambdalambda1031.93, 1037.62 absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the velocity range from -100 to 100 km s(-1) reveals a wide-spread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T similar to 3 x 10(5) K in the Milky Way thick disk/halo. The integrated column density, log [N(O VI) cm(-2)], ranges from 13.85 to 14.78 with an average value of 14.38 and a standard deviation of 0.18. Large irregularities in the gas distribution are found to be similar over angular scales extending from <1 degrees to 180 degrees, implying a considerable amount of small- and large-scale structure in the absorbing gas. The overall distribution of O vi is not well described by a symmetrical plane-parallel layer of patchy O VI absorption. The simplest departure from such a model that provides a reasonable fit to the observations is a plane-parallel patchy absorbing layer with an average O VI midplane density of n(0)(O VI) = 1.7 x 10(-8) cm(-3), a scale height of similar to 2.3 kpc, and a similar to 0.25 dex excess of O VI in the northern Galactic polar region. The distribution of O VI over the sky is poorly correlated with other tracers of gas in the halo, including low- and intermediate-velocity H I, H alpha emission from the warm ionized gas at similar to 10(4) K, and hot X-ray-emitting gas at similar to 10(6) K. The O vi has an average velocity dispersion, b approximate to 60 km s(-1), and standard deviation of 15 km s(-1). Thermal broadening alone cannot explain the large observed pro. le widths. The average O VI absorption velocities toward high-latitude objects (\b\ > 45degrees) range from -46 to 82 km s(-1), with a high-latitude sample average of 0 km s(-1) and a standard deviation of 21 km s(-1). High positive velocity O VI absorbing wings extending from similar to100 to similar to250 km s(-1) observed along 21 lines of sight may be tracing the flow of O VI into the halo. A combination of models involving the radiative cooling of hot fountain gas, the cooling of supernova bubbles in the halo, and the turbulent mixing of warm and hot halo gases is required to explain the presence of O VI and other highly ionized atoms found in the halo. The preferential venting of hot gas from local bubbles and superbubbles into the northern Galactic polar region may explain the enhancement of O VI in the north. If a fountain flow dominates, a mass flow rate of approximately 1.4 M. yr(-1) of cooling hot gas to each side of the Galactic plane with an average density of 10(-3) cm(-3) is required to explain the average value of log [N(O VI) sin \b\] observed in the southern Galactic hemisphere. Such a flow rate is comparable to that estimated for the Galactic intermediate-velocity clouds.