Is it possible to turn an elliptical radio galaxy into a BL Lac object?

The space density of BL Lac objects having extended emission with luminosity $$\geqslant 10^{22}\enspace \text W\enspace \text {Hz}^{-1}\enspace\text {sr}^{-1}$$ at 1.4 GHz is estimated and compared with the space density of elliptical radio galaxies with the same luminosity. It is argued that, if the characteristic properties of BL Lacs are a result of relativistic beaming, then elliptical radio galaxies are the unaligned parent population of the BL Lac objects. The relative space densities constrain the Lorentz factor of the beams in BL Lacs (and radio galaxies) to be ≲ 5.4. Lorentz factors of ≳ 4 are sufficient to boost the weak central component of an elliptical radio galaxy into the dominant core of a BL Lac object.