Testing the FR I/BL Lac unifying model with HST observations

Hubble Space Telescope (HST) observations provide a novel way of testing un ified models for FR I radio sources and BL Lac objects. The detection of ex tended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predictions. As a pilot project towards using HST information in testing unified models, we selected five radio galaxies that show extended nuclear discs in the HS T images. The relative orientation of the projected radio jets and of the e xtended nuclear discs indicates that they are not perpendicular, as the sim plest geometrical model would suggest, but that they form an angle of simil ar to 20-40 degrees with the symmetry axis of the disc: a significant chang e of orientation occurs between the innermost AGN structure and the kilopar sec scale. Nevertheless, the discs appear to be useful indicators of the or ientation of the radio source, since the angles formed by the disc axis and the jet with the line of sight differ by only similar to 10-20 degrees. At the centre of each disc an unresolved nuclear source is present. We comp ared its luminosity with the optical core luminosity of BL Lacs selected fo r having similar host galaxy magnitude and extended radio luminosity. The B L Lac cores are between 2 x 10(2) and 3 x 10(5) times brighter than the cor responding radio galaxy ones. The FR I/BL Lac core luminosity ratio shows a suggestive correlation with t he orientation of the radio galaxies with respect to the line of sight. The behaviour of this ratio is quantitatively consistent with a scenario in wh ich the emission in the FR I and BL Lac sources is dominated by the beamed radiation from a relativistic jet with Doppler factor similar to 5-10, thus supporting the basic features of the proposed unification schemes. Several observational tests, based on the method proposed here, can strengt hen our conclusions and improve the statistical significance of the finding s presented.