8.1.4 The Parent Population of BL Lac Objects
There are some potential mismatches in the properties of FR I radio galaxies and BL Lac objects, although these are far from demonstrated conclusively. First, the narrow emission line strengths of BL Lacs, where measured, barely overlap with the distribution for FR Is (Sec. 5.4.2, Fig. 11), although this is complicated by the lack of complete samples and uniform coverage, not to mention the extreme difficulty of detecting very weak lines (and the lack of published upper limits) in BL Lac objects.
Second, broad Mg II lines are quite strong in some high-redshift BL Lacs (Sec. 7.1, Fig. 19) while there is little evidence for similarly strong broad lines in nearby FR Is. In part this could be due to the lack of sensitive UV measurements of FR Is or to a correlation with continuum luminosity (and therefore redshift); both effects would limit the observation of Mg II in existing, very local, samples of FR Is.
Third, the diffuse radio emission of BL Lac objects displays FR II morphology in some cases (Sec. 5.4.1) and sometimes quite high unbeamed luminosity (Fig. 10). There may also be a deficit of twin-jet morphologies in BL Lacs, depending on the true value of the critical angle. It may well be that the parent population of BL Lac objects includes at least some FR II radio galaxies, presumably those with low-excitation optical spectra. We expect that the beaming calculation under this hypothesis would show this scheme to be viable and that the derived Lorentz factor(s) will not change much. Certainly this would be an interesting calculation to do once the optical spectra of the 2 Jy sample are sufficient to separate the low-excitation and high-excitation FR IIs.