A comparison of the optical properties of radio-loud and radio-quiet quasars

We have made radio observations of 87 optically selected quasars at 5 GHz w ith the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently complete d Edinburgh Quasar Survey and covers a redshift range of 0.3 less than or e qual to z less than or equal to 1.5 and an optical absolute magnitude range of -26.5 less than or equal to M-B less than or equal to -23.5 (h = 1/2, q (0) = 1/2). We have also matched other existing surveys with the Faint Imag es of the Radio Sky at Twenty Centimeters and NRAO VLA Sky Survey radio cat alogs and combined these data so that the optical luminosity-redshift plane is now far better sampled than before. We have fitted a model to the proba bility of a quasar being radio-loud as a function of absolute magnitude and redshift, and from this model we infer the radio-loud and radio-quiet opti cal luminosity functions. The radio-loud optical luminosity function is fea tureless and flatter than the radio-quiet one. It evolves at a marginally s lower rate if quasars evolve by density evolution, but the difference in th e rate of evolutions of the two different classes is much less than was pre viously thought. We show, using Monte Carlo simulations, that the observed difference in the shape of the optical luminosity functions can be partly a ccounted for by Doppler boosting of the optical continuum of the radio-loud quasars, and we explain how this can be tested in the future.