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2001ApJ...549..281R - Astrophys. J., 549, 281-292 (2001/March-1)

The FUSE spectrum of PG 0804+761: a study of atomic and molecular gas in the lower galactic halo and beyond.


Abstract (from CDS):

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 (zem=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 zabs=0.019 and absorption by H I, C III, and O VI in an associated system at zabs=0.102. No intervening O VI absorbers are seen in the spectrum of PG 0804+761.

Abstract Copyright:

Journal keyword(s): Galaxy: Halo - ISM: Abundances - ISM: Clouds - Galaxies: Quasars: Absorption Lines - Galaxies: Quasars: Individual: Alphanumeric: PG 084+761 (read: PG 0804+761)

Simbad objects: 18

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