The radio-loud/radio-quiet dichotomy: news from the 2dF QSO Redshift Survey

We present a detailed analysis of a sample of radio-detected quasars, obtained by matching together objects from the FIRST survey and the 2dF Quasar Redshift Survey (2dF). The data set consists of 113 sources, spanning a redshift range 0.35less than or similar tozless than or similar to2.2, with optical magnitudes 18.25less than or equal tob(J)less than or equal to20.85 and radio fluxes S(1.4 GHz)greater than or equal to1 mJy. These objects exhibit properties such as redshift and colour distribution in full agreement with those derived for the whole quasar population, suggesting that the mechanism(s) controlling the birth and lifetime of quasars are independent of their level of radio emission. The long-debated question of the radio-loud/radio quiet (RL/RQ) dichotomy is then investigated for the combined FIRST-2dF and FIRST-LBQS (Large Bright Quasar Survey) samples, as they present similar selection criteria. We find the fraction of radio detections to increase with magnitude from less than or similar to3 per cent at the faintest levels up to similar to20 per cent for the brightest sources. The classical RL/RQ dichotomy, in which the distribution of radio-to-optical ratios and/or radio luminosities shows a lack of sources, is ruled out by our analysis. We also find no tight relationship between optical and radio luminosities for sources in the sample considered, a result that tends to exclude the mass of the quasar black hole as the physical quantity determining the level of radio emission.