Far Ultraviolet Spectroscopic Explorer Observations of O VI Absorption in the Galactic Halo

Far-ultraviolet spectra of 11 active galactic nuclei observed by Far Ultraviolet Spectroscopic Explorer (FUSE) are analyzed to obtain measures of O VI λ1031.93 absorption occurring over very long paths through Milky Way halo gas. Strong O VI absorption is detected along 10 of 11 sight lines. Values of log[N(O VI) sin |b|] range from 13.80 to 14.64, with a median value of 14.21. The observations reveal the existence of a widespread but irregular distribution of O VI in the Milky Way halo. Combined with estimates of the O VI midplane density, n₀ = 2 × 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 ~0.15 in the disk to ~0.6 along four extragalactic sight lines. The changing ionization state of the gas from the disk to the halo is consistent with a systematic decrease in the scale heights of Si IV, C IV, N V, to O VI from ~5.1 to ~2.7 kpc. While conductive heating models can account for the highly ionized atoms at low |z|, a combination of models (and 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 turbulent mixing layers or by photoionization by hot halo stars or the extragalactic background.