Observations of O VI emission from the diffuse interstellar medium

We report the first Far-Ultraviolet Spectroscopic Explorer measurements of diffuse O VI (lambda lambda 1032, 1038) emission from the general diffuse i nterstellar medium outside of supernova remnants or superbubbles. We observ ed a 30 " x30 " region of the sky centered at l=315 degrees .0 and b = -41 degrees .3. From the observed intensities (2930 +/- 290 [random] +/- 410 [s ystematic] and 1790 +/- 260 [random] +/- 250 [systematic] photons cm(-2) s( -1) sr(-1) in lambda lambda 1032 and 1038, respectively), derived equations , and assumptions about the source location, we calculate the intrinsic int ensity, electron density, thermal pressure, and emitting depth. The intensi ties 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 contri bute. High-velocity clouds are ruled out because there are none near the po inting direction. The calculated emitting depth is small, indicating that t he 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 constra in models. The data also yield the first intensity measurement of the C II 3s S-2(1/2)-2p P-2(3/2) emission line at 1037 Angstrom and place upper limi ts on the intensities of ultraviolet line emission from C I, CIII, Si II, S III, S IV, S VI, and Fe III.