ITAL]ORFEUS II[/ITAL] Far-Ultraviolet Observations of 3C 273: Interstellar and Intergalactic Absorption Lines

Abstract

We present the first intermediate-resolution (λ/Δλ = 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 A. Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyβ counterparts to intergalactic Lyα features identified by Morris and coworkers at cz = 19,900, 1600, and 1000 km s−1; counterparts to other putative Lyα clouds along the sight line are below our detection limit. The strengths of the two very low redshift Lyβ features, which are believed to arise in Virgo intracluster gas, exceed preflight expectations (Weymann and coworkers), suggesting that the previous determination of the cloud parameters may underestimate the true column densities. A curve-of-growth analysis sets a minimum H I column density of 4 × 1014 cm−2 for the 1600 km s−1 cloud. We find marginally significant evidence for Galactic H2 along the sight line, with a total column density of about 1015 cm−2. We detect the stronger interstellar O VI doublet member unambiguously; the weaker member is blended with other features. If the Doppler b-value for O VI is comparable to that determined for N V by Sembach and collaborators, then the O VI column density is (7 ± 2) × 1014 cm−2, significantly above the only previous estimate, by Davidsen. The O VI/N V ratio is about 10, consistent with the low end of the range observed in the disk, as shown in the compilation by Hurwitz & Bowyer. Additional interstellar species detected for the first time toward 3C 273 (at modest statistical significance) include P II, Fe III, Ar I, and S III.