The FUSE Survey of Diffuse O VI Emission from the Interstellar Medium

Emission via the O VI doublet at 1031.93 and 1037.62 A is the primary cooling mechanism for interstellar gas at temperatures between 10^5 and 10^6 K. As such, it traces regions where hot gas cools quiescently, interface regions where hot and cool gas meet and mingle, and high-velocity shocks. Over its eight-year mission, the Far Ultraviolet Spectroscopic Explorer (FUSE) was used to observe O VI in both emission and absorption. In this talk, I will present a brief history of the hot component of the interstellar medium (ISM) and the various physical processes responsible for its creation, evolution, and morphology. I will review the results of recent O VI absorption- line studies of the Galactic disk and halo. Finally, I will present the latest results from our survey of diffuse O VI emission from the ISM.

Our observations reveal the large-scale structure of the O VI-emitting gas in the quadrant of the sky centered on the Magellanic Clouds. Its most prominent feature is a layer of low-velocity emission that extends more than 70 degrees from the Galactic plane. By combining absorption and emission measurements along the same lines of sight, we can estimate the emission measure and therefore the density of the emitting gas. We identify two distinct populations of emitting clouds: At low latitudes (|b| < 30 deg), we probe narrow, high-
density conductive interfaces in the local ISM. At high latitudes, the low densities and long path lengths are consistent with a location in the Galactic thick disk/halo. Three of our sight lines exhibit O VI emission at velocities consistent with Magellanic H I, suggesting that hot and cool gas is mixing in the interface region of the Magellanic System.

Dr. Van Dixon

Johns Hopkins University