An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium

We present a survey of diffuse O VI emission in the interstellar medium (ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Spanning 5.5 yr of FUSE observations, from launch through 2004 December, our data set consists of 2925 exposures along 183 sight lines, including all of those with previously published O VI detections. The data were processed using an implementation of CalFUSE version 3.1 modified to optimize the signal-to-noise ratio and velocity scale of spectra from an aperture-filling source. Of our 183 sight lines, 73 show O VI λ1032 emission, 29 at >3 σ significance. Six of the 3 σ features have velocities |vLSR| > 120 km s-1, while the others have |vLSR| ≤ 50 km s-1. Measured intensities range from 1800 to 9100 LU (line unit; 1 photon cm-2 s-1 sr-1), with a median of 3300 LU. Combining our results with published O VI absorption data, we find that an O VI-bearing interface in the local ISM yields an electron density ne = 0.2-0.3 cm-3 and a path length of 0.1 pc, while O VI-emitting regions associated with high-velocity clouds in the Galactic halo have densities an order of magnitude lower and path lengths 2 orders of magnitude longer. Although the O VI intensities along these sight lines are similar, the emission is produced by gas with very different properties.