The FUSE spectrum 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 i n the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the low-red shift quasar PG 0804 + 761 (z(em) = 0.100) at intermediate resolution (FWHM similar to 25 km s(-1)) in the direction l = 138.degrees3, b = 31.degrees0 . With a good signal-to-noise ratio (S/N) and the presence of several inter esting Galactic and extragalactic absorption components along the sight lin e, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectr um 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 hydroge n 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 abs orption is found in six lines for rotational levels 0 and 1, leading to a t otal H-2 column density of log N = 14.71 +/- 0.30. We derive an O I gas-pha se abundance for the IVC of 1.03(-0.42)(+0.71) solar. Lower abundances of o ther elements (Fe, Si) imply depletion onto dust grains or the presence of higher, undetected ionization states. The presence of N II and Fe III absor ption 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 ioniz ation H+/(H-0+H+) of similar to 19%. Absorption by O VI is found at velocit ies as negative as -110 km s(-1), but no absorption from any species is fou nd at velocities of similar to -180 km s(-1), where absorption from the nea rby high-velocity cloud complex A would be expected. We suggest that the ex tended O VI absorption traces hot gas situated above the Perseus spiral arm . Finally, we find intergalactic absorption by an intervening H I Ly beta a bsorber at z(abs) = 0.019 and absorption by H I, C III, and O VI in an asso ciated system at z(abs) = 0.102. No intervening O VI absorbers are seen in the spectrum of PG 0804+761.