BL Lacertae objects and radio-loud quasars within an evolutionary unified scheme
F. Vagnetti, A. Cavaliere, E. Giallongo · The Astrophysical Journal · 1991
On the basis of a joint radio-optical analysis, we find different amounts of cosmological luminosity evolution in the optical band for flat- and steep-spectrum radio-loud quasars. Specifically, we find a slower evolution for the flat-spectrum quasars, evidence of a "sluggish" beamed component in the optical band additional to the canonical "isotropic" continuum. We interpret the persistence of the beamed component in terms of a spectral evolution with the Doppler factor increasing somewhat in cosmic time. We place the BL Lacertae objects into this framework and propose an evolutionary unified scheme maintaining spectral and statistical continuity of BL Lacs with flat- spectrum quasars. Supportive observational evidence, and clues for the parent population, are discussed.