BL Lacertae objects as an evolutionary population

Abstract

If the BL Lac Objects are active nuclei with a beamed component that is dominant when directed at us, their observed luminosity function must comprise a flat faint branch: N(L)dL ∝ L1+1/pdL with p=4.5 (Urry and Shafer 1984). If this is flatter than the LF NP(L) of the parent objects at equal observed L, then we expect the counts of BL Lacs to flatten out in turn at fluxes quite higher than the counts of the parents, even when both populations evolve strongly and uniformly with comparable timescales (Cavaliere, Giallongo and Vagnetti 1985).