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Title:
The FUSE Spectrum of PG 0804+761: A Study of Atomic and Molecular Gas in the Lower Galactic Halo and Beyond
Authors:
Richter, Philipp; Savage, Blair D.; Wakker, Bart P.; Sembach, Kenneth R.; Kalberla, Peter M. W.
Affiliation:
AA(Washburn Observatory, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706; , , ), AB(Washburn Observatory, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706; , , ), AC(Washburn Observatory, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706; , , ), AD(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218; ), AE(Universität Bonn, Radioastronomisches Institut, D-53121 Bonn, Germany; )
Publication:
The Astrophysical Journal, Volume 549, Issue 1, pp. 281-292. (ApJ Homepage)
Publication Date:
03/2001
Origin:
UCP
ApJ Keywords:
Galaxy: Halo, ISM: Abundances, ISM: Clouds, Galaxies: Quasars: Absorption Lines, Galaxies: Quasars: Individual: Alphanumeric: PG 084+761
DOI:
10.1086/319070
Bibliographic Code:
2001ApJ...549..281R

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 (zem=0.100) at intermediate resolution (FWHM~25 km s-1) in the direction l=138.3d, b=31.0d. 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.
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