Ultraviolet polarimetry and spectroscopy of the BL lacertae object PKS 2155-304

We present ultraviolet spectropolarimetry of the BL Lac object PKS 2155-304. These observations were made using the Hubble Space Telescope and the Faint Object Spectrograph and represent the first opportunity to study the wavelength dependence of the UV (1360-3300 Å) linear polarization in a BL Lac object. Nearly simultaneous ground-based optical polarization and flux measurements were also taken of PKS 2155-304. During these observations the object was faint (V = 13.7-13.8) and lowly polarized (P = 2%-4%). The UV/optical polarization increased smoothly by about a factor 2 from 8000 Å to ∼ 1400 Å. This wavelength dependence in the polarization is stronger than typically observed for this object and other BL Lac objects; however, the plane of polarization was constant with wavelength. The UV polarimetry yields strong evidence that the UV polarized flux is produced by the same synchrotron mechanism that is responsible for the optical polarization, and that both the UV and optical emission are produced in the same region of the source. There is also no evidence from these measurements to support models of PKS 2155-304 that have an accretion disk or other thermal source contributing significantly to its UV continuum. We also present an ultraviolet spectrum from 1150 Å to 1600 Å of PKS 2155-304. Absorption lines from the Galactic halo are present, but we also find several prominent absorption lines which are not obviously associated with the halo or with the object, thought to be at z ∼ 0.1. The absorptions are consistent with strong (up to 0.8 Å rest equivalent width) Lyα forest lines from intervening clouds, similar to those recently found in HST spectra of 3C 273. Thus our observations both confirm and exacerbate the surprise that Lyα absorption systems are present at low redshifts.