2001ApJ...549..281R

We present an analysis of interstellar and intergalactic absorption lines in the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the low-redshift quasar PG 0804+761 (z_em=0.100) at intermediate resolution (FWHM∼25 km.s^–1) in the direction l=138°.3, b=31°.0. With a good signal-to-noise ratio (S/N) and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG 0804+761 is dominated by strong absorption from local Galactic gas at 0 km.s^–1, we concentrate our study on absorption by molecular hydrogen and neutral and ionized metals related to an intermediate-velocity cloud (IVC) in the lower Galactic halo at -55 km.s^–1, and on absorption from O VI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in six lines for rotational levels 0 and 1, leading to a total H₂ column density of log N=14.71±0.30. We derive an O I gas-phase abundance for the IVC of 1.03^+0.71_–0.42 solar. Lower abundances of other elements (Fe, Si) imply depletion onto dust grains or the presence of higher, undetected ionization states. The presence of N II and Fe III absorption at -55 km.s^–1 indicates that a fraction of the hydrogen is ionized. From the relative abundances of O I and P II we estimate a degree of ionization H⁺/(H⁰+H⁺) of ∼19%. Absorption by O VI is found at velocities as negative as -110 km.s^–1, but no absorption from any species is found at velocities of ~-180 km.s^–1, where absorption from the nearby high-velocity cloud complex A would be expected. We suggest that the extended O VI absorption traces hot gas situated above the Perseus spiral arm. Finally, we find intergalactic absorption by an intervening H I Lyβ absorber at z_abs=0.019 and absorption by H I, C III, and O VI in an associated system at z_abs=0.102. No intervening O VI absorbers are seen in the spectrum of PG 0804+761.