TheFUSESpectrum of PG 0804+761: A Study of Atomic and Molecular Gas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines inthe FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) atintermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With agood signal-to-noise ratio and the presence of several interesting Galactic andextragalactic absorption components along the sight line, this spectrumprovides a good opportunity to demonstrate the ability of FUSE to do bothinterstellar and extragalactic science. Although the spectrum of PG0804+761 isdominated by strong absorption from local Galactic gas, we concentrate ourstudy on absorption by molecular hydrogen and neutral neutral and ionizedmetals related to an intermediate-velocity cloud in the lower Galactic halo at-55 km/s, and on absorption from OVI extended to negative velocities. In theIVC, weak molecular hydrogen absorption is found in 5 lines for rotationallevels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30).We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar.Absorption by OVI is found at velocities as negative as -110 km/s, but noabsorption from any species is found at velocities of -180 km/ where absorptionfrom the nearby high-velocity Complex A would be expected. We suggest that theextended OVI absorption traces hot gas above the Perseus spiral arm. Finally,we find intergalactic absorption by an intervening HI Ly betax absorber atz=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102.No intervening OVI absorbers are seen in the spectrum of PG0804+761.Comment: 27 pages, 6 figures; accepted for publication in Ap