Medium-Resolution Far-Ultraviolet Spectroscopy of PKS 2155-304

Abstract

Using the Berkeley spectrometer of the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) we observed the 87-117 nm UV spectrum of the BL Lac object PKS 2155 - 304 with about 0.5 A resolution. In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Lyα lines discovered earlier with IUE and the HST in the direction of PKS 2155-304. The Lyman discontinuities indicate for three of the redshifted clouds a combined H I column density of 2-5 x 10^16^ cm^-2^, while the column density for another cloud appears to be well below 5 x 10^15^ cm^-2^. No significant O VI absorption in the galactic halo toward PKS 2155 - 304 could be detected from our data. Assuming that saturation effects are negligible for these weak features, we obtain for the O VI column density toward PKS 2155-304 a 3 σ upper limit of 2.7 x 10^14^cm^-2^.