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

Abstract

We present the first intermediate-resolution (l/Dl ϭ 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 Å . Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyb counterparts to intergalactic Lya features identified by Morris and coworkers at cz ϭ 19,900, 1600, and 1000 km s ; counterparts to other putative Lya clouds along the sight Ϫ1 line are below our detection limit. The strengths of the two very low redshift Lyb 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 # 10 14 cm for the 1600 km s cloud. We find marginally Ϫ2 Ϫ1 significant evidence for Galactic H along the sight line, with a total column density of about 10 15 cm . We Ϫ2 2 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 ‫ע‬ 2) # 10 14 cm , significantly above the only previous estimate, by Davidsen. Ϫ2The 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.

A high-resolution study of the evolution of the Lyman α forest in the redshift interval $0.9 < z < 1.7$

“…Figure 1 shows a rather small systematic effect in the column densities. We derive N e.g., Hurwitz et al 1998;Shull et al 2000). Our 15 per cent deviation of the inferred column densities is at variance with the above cited studies.…”

[ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of O [CSC]vi[/CSC] Absorption in the Galactic Halo

“…This high-velocity O vi absorption is likely produced by differential Galactic rotation, which causes the absorption to extend to large negative velocities. Table 1 along with the ORFEUS value for 3C 273 from Hurwitz et al (1998). The large value (14.80) for the 3C 273 sight line is likely a consequence passing through Radio Loops I and IV and the North Polar Spur.…”

“…Except for brief observing programs by the Hopkins Ultra- Hurwitz & Bowyer 1995;Hurwitz et al 1998;Widmann et al 1998;Sembach, Savage, & Hurwitz 1999), we have had a long hiatus in observing O vi. This has ended with the commissioning of the Far Ultraviolet Spectroscopic Explorer (FUSE), a facility that has been specially designed to cover wavelengths from 905 to 1187 Å with an efficient two-dimensional detector.…”

The Local Lyα Forest. IV. Space Telescope Imaging Spectrograph G140M Spectra and Results on the Distribution and Baryon Content of H i Absorbers

“…It has been shown that the only reliable method of deriving b-values for partially saturated Ly forest lines is by a curve of growth using higher Lyman series lines (Hurwitz et al 1998;Shull et al 2000). Significantly lower b-values (factor of 2 in the median) are found from the curve-of-growth technique (Shull et al 2000).…”