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The distribution of QSO radio luminosities has long been debated in theliterature. Some argue that it is a bimodal distribution, implying that thereare two separate QSO populations (normally referred to as 'radio-loud' and'radio-quiet'), while others claim it forms a more continuous distributioncharacteristic of a single population. We use deep observations at 20 GHz toinvestigate whether the distribution is bimodal at high radio frequencies.Carrying out this study at high radio frequencies has an advantage overprevious studies as the radio emission comes predominantly from the core of theAGN, hence probes the most recent activity. Studies carried out at lowerfrequencies are dominated by the large scale lobes where the emission is builtup over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Oursample comprises 874 X-ray selected QSOs that were observed as part of the 6dFGalaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of0.2-0.5 mJy.  No evidence of bimodality is seen in either the 20 GHz luminositydistribution or in the distribution of the R_20 parameter: the ratio of theradio to optical luminosities traditionally used to classify objects as beingeither radio-loud or radio-quiet. Previous results have claimed that at lowradio luminosities, star formation processes can dominate the radio emissionobserved in QSOs. We attempt to investigate these claims by stacking theundetected sources at 20 GHz and discuss the limitations in carrying out thisanalysis. However, if the radio emission was solely due to star formationprocesses, we calculate that this corresponds to star formation rates rangingfrom ~10 solar masses/yr to ~2300 solar masses/yr.

Is the observed high-frequency radio luminosity distribution of QSOs bimodal?