The radio and optical properties of the z < 0.5 BQS quasars

Abstract

We report on a comprehensive study of the radio properties of the 89 low- redshift (z < 0.5) BQS quasars. High-resolution radio maps of the 28 objects that show resolved structure are presented, resulting principally from a re-analysis of observations made at 5 GHz by Kellermann et al., but supplemented by new observations at 1.5 and 5 GHz. We compile a list of the best available radio and optical positions and radio flux densities, and classify their radio structures. By studying the radio–[O III] plane, we find that the radio-loud objects exist only at high [O III] luminosities, that the radio-quiet objects show a strong radio–[O III] correlation, and that the few objects of intermediate radio luminosity occupy anomalous regions of the plane. The use of statistical methods leads to an interpretation of the different structural classes of radio-quiet quasar (RQQ) which requires contributions from a circumnuclear starburst both to the radio and to the [O III] emission. In unifying the radio-quiet quasars with those of intermediate radio luminosity by means of relativistic beaming, we find that a common feature must be relativistic nuclear jets which have γ ~ 5 but have bulk kinetic powers much lower than those of the jets of radio galaxies. Simulations are carried out which show that a model with an average of 20 per cent of [O III] emission and 50 per cent of radio emission coming from a starburst can mimic the radio–[O III] relations of the RQQs, while beaming of about 10 per cent of the nuclear radio emission can give rise to a few objects of enhanced radio luminosity. Although trends in the relation between radio and Fe II emission in the sample do not support a model of jet-induced Fe II emission, they are consistent with an orientation model, but the evidence for this latter model is dominated by two anomalous groups of objects in which geometrical differences must also exist.