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

Abstract

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 at intermediate (z em 0.100) resolution (FWHM D 25 km s~1) in the direction With a good signal-to-noise ratio l 138i.3, b 31i.0. (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 column density of log N 14.71 ^ 0.30. We derive an O I gas-phase abundance for the IVC of H 2 solar. Lower abundances of other elements (Fe, Si) imply depletion onto dust grains or the 1.03 ~0.420.71 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‘ )o fD19%. 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 D[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 —nd intergalactic absorption by an intervening H I Lyb absorber at