Fanaroff-Riley I galaxies as the parent populations of BL Lacertae objects. III - Radio constraints

There is increasing support for the idea that BL Lacertae objects are dominated by beamed emission from a relativistic jet aligned with the line of sight. If so, the most likely parent objects are Fanaroff-Riley type I (F-R I) radio galaxies. This hypothesis is tested by determining the ratio of number densities of the two populations in a manner appropriate for flux-limited samples. Through a direct comparison of predicted and observed luminosity functions, it is shown that available radio samples of F-R I galaxies and BL Lac objects are fully consistent with the beaming hypothesis. The model makes specific predictions about the luminosity function of BL Lacertae objects and their radio counts below currently observed levels.