The Hot Phase in the Galactic Disk

Spectra of about 150 early-type stars in the disk of the Milky Way recorded by the Far Ultraviolet Spectroscopic Explorer (FUSE) reveal absorption features from interstellar O VI, a tracer of collisionally ionized gases at temperatures near 300,000 K. (Hotter material is better sensed by its diffuse x-ray emission.) The FUSE survey indicates that hot gas in the plane of the Galaxy yields an average density n(O VI) = 1.7 X 10^{-8} cm^{-3}, a value consistent with earlier results from the Copernicus satellite, but at great distances there is a large dispersion in the measurements from one line of sight to another. This dispersion indicates that O VI absorbing regions probably have a broad distribution of sizes, perhaps giving a power-law behavior for column densities. Comparisons of O VI with other Li-like ions should give some guidance on the nature and origin of the O VI-absorbing regions, but poor correspondences in the velocity profile shapes will complicate the interpretations.