MEDIUM-RESOLUTION FAR-ULTRAVIOLET SPECTROSCOPY OF PKS-2155-304

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 Angstrom resolution. In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Ly alpha 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 sigma upper limit of 2.7 x 10(14) cm(-2).