A Hole in the Galaxy's Emissive O VI Coverage

We report observations of O VI resonance line emission (1032, 1038 A) along the sight line to NGC 1068 (M77, l = 172.1, b = −51.9), made using the Far Ultraviolet Spectroscopic Explorer’s (FUSE’s) low resolution aperture (LWRS). The observations place tight upper limits on the O VI emission originating in our galaxy’s diffuse interstellar medium as well as tight upper limits on the emission originating in NGC 1068’s extended disk. The Milky Way’s intensities in the O VI 1032 and 1038 A emission lines are 150 ± 310 and -70 ± 280 photons cm s sr, respectively. These values are an order of magnitude less than those reported for other unobscured, high latitude sight lines, indicating a hole in the distribution of emissive interstellar O VI ions. Nonetheless, extensive O VI column density surveys have found large O VI column densities on almost all high S/N extragalactic lines of sight, including a line of sight only 2 arcminutes from our pointing direction. From the IO V I/NOV I ratio, we calculate the electron density, ne. Either it is extremely low (ne < 0.003 cm ) or the O VI-rich plasma is much cooler than its collisional ionizational equilibrium temperature. It’s character must differ from the emissive O VI seen toward other high latitude directions. We also made the first search for O VI emission from the disk of a Seyfert galaxy, using NGC 1068. The intensities in the redshifted O VI emission lines (1036 and 1042 A) are 10 ± 290 and 160 ± 330 photons cm s sr, respectively. These are surprisingly tight upper limits for a galaxy known to have starburst activity in its disk and an AGN in its center. This work was supported by NASA, through grant numbers NNG04GN77G and NNG04GD78G.