Detection of Extragalactic Absorption in the Ultraviolet Spectrum of the BL Lacertae Object PKS 2155-304

Abstract

The complete collection of ultraviolet spectra (1200-3000 A) of PKS 2155-304 taken with IUE in l979-1986 is reanalyzed with the aim of producing a high signal-to-noise ratio spectrum. As a result of co-adding the best exposed spectra a number of absorption lines are found, most of which can be identified with expected interstellar species in our Galaxy, and show equivalent widths similar to those measured in the direction of the Magellanic Clouds. Some absorption lines however remain unidentified, in particular a prominent feature at 1285 A, which is the strongest line in the short-wavelength spectrum. We consider possible origins for these lines. The most plausible is to interpret λ1285 as intervening Lyα at z = 0.057, with another unidentified line attributable to C II λ1334 at the same redshift. This system could be associated with a possible intervening galaxy. Alternatively the Lyα absorption could be truly intergalactic, in analogy with the Lyα forest observed in high redshift quasars.