Optical spectroscopy of radio galaxies in abell clusters .2. BL Lacs and FR I unification

As part of our optical spectroscopic study of radio galaxies in Abell clusters, we report a search for BL Lac phenomena in our sample, As an indicator of possible nonthermal activity in the nuclei of our radio galaxies, we report D(4000), a measure of the 4000 Angstrom break, for 190 objects. None of the objects in our sample are found which meet the standard definitions for BL Lac objects both for D(4000) and for weakness of emission lines. We do find 15 objects which have diluted 4000 Angstrom breaks [D(4000)less than or equal to 1.8], and these are discussed individually. At least four of them appear to have weak nonthermal activity which may be related to the BL Lac phenomena. These results appear to be in conflict with the simplest BL Lac/FR I unification model. Several ways out of this dilemma are discussed. Perhaps the most likely change in this model is the inclusion of FR II radio galaxies as part of the parent population of BL Lac objects. (C) 1996 American Astronomical Society.