Abstract

The properties of two complete samples of flat-spectrum radio quasars and radio-selected BL Lacs are analysed to look for any relationship between the two classes. It is shown that BL Lacs are not quasars with emission lines swamped by an enhanced optical continuum but their line luminosities are intrinsically weak. Moreover, an evolutionary connection between the two classes does not seem to be supported by the present data, while micro-lensing of quasars by stars in foreground galaxies can be ruled out as an explanation for the BL Lac phenomenon. BL Lacs and flat-spectrum radio quasars probably represent separate instances of relativistic beaming in low- and high-luminosity radio galaxies respectively.

Introduction

BL Lacertae objects are special types of active galactic nuclei, characterized by rapid variability, relatively high optical polarization, flat radio spectrum, radio coredominance, superluminal motion, and weak or absent emission lines. Flat-spectrum radio quasars (FSRQs) are quasars with spectral index α ≤ 0.5 (Fv ∝ v−α) at a few GHz, in many ways similar to BL Lacs (members of the two classes are often grouped together under the blazar category). The most striking difference between BL Lacs and FSRQs is the presence of strong emission lines in the latter objects. Although there are undoubtedly some borderline objects in which emission lines appear when the continuum is in a low state, a rest-frame equivalent width of 5 Å seems to separate quite well the two classes (see discussion in Stickel et al. 1991).