HST observations of FR I radiogalaxies: what do they tell us about the BL Lac - FR I unification scheme?

Abstract

We explore the viability of the unification of BL Lacs and FR I radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are studied in the radio-optical luminosity plane: we calculate the predicted luminosity of BL Lacs when observed off-axis, in the frame of a simple one-zone model. We find that the bulk Lorentz factors required in order to account for the observed luminosities are significantly smaller than those implied by other, both observational and theoretical, considerations. In order to reconcile these results with the unification scheme, velocity structures in the jet are suggested, where a fast spine is surrounded by a slower layer.