TheFar Ultraviolet Spectroscopic ExplorerSurvey of OviAbsorption in the Disk of the Milky Way

Abstract

To probe the distribution and physical characteristics of interstellar gas at temperatures T ≈ 3 × 10⁵ K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of O VI λ1032 toward 148 early-type stars situated at distances >1 kpc. After subtracting off a mild excess of O VI arising from the Local Bubble, combining our new results with earlier surveys of O VI, and eliminating stars that show conspicuous localized X-ray emission, we find an average O VI midplane density n₀ = 1.3 × 10⁻⁸ cm⁻³. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of O VI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that O VI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(O VI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the O VI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.