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

Hubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended 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 HST images. The relative orientation of the projected radio jets and of the extended nuclear discs indicates that they are not perpendicular, as the simplest geometrical model would suggest, but that they form an angle of similar to 20-40 degrees with the symmetry axis of the disc: a significant change of orientation occurs between the innermost AGN structure and the kiloparsec scale. Nevertheless, the discs appear to be useful indicators of the orientation 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 compared its luminosity with the optical core luminosity of BL Lacs selected for having similar host galaxy magnitude and extended radio luminosity. The BL Lac cores are between 2 x 10(2) and 3 x 10(5) times brighter than the corresponding radio galaxy ones. The FR I/BL Lac core luminosity ratio shows a suggestive correlation with the orientation of the radio galaxies with respect to the line of sight. The behaviour of this ratio is quantitatively consistent with a scenario in which 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 strengthen our conclusions and improve the statistical significance of the findings presented.