Observations of O VI Emission from the Diffuse Interstellar Medium

We report the first Far-Ultraviolet Spectroscopic Explorer measurements of diffuse O VI (λλ1032, 1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30′′ × 30′′ region of the sky centered at l = 315.°0 and b = -41.°3. From the observed intensities (2930 ± 290 [random] ± 410 [systematic] and 1790 ± 260 [random] ± 250 [systematic] photons cm-2 s-1 sr-1 in λλ1032 and 1038, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High-velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and to constrain models. The data also yield the first intensity measurement of the C II 3s 2S1/2-2p 2P3/2 emission line at 1037 Å and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III.