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We investigate the dichotomy in the radio loudness distribution of quasars bymodelling their radio emission and various selection effects using a MonteCarlo approach. The existence of two physically distinct quasar populations,the radio-loud and radio-quiet quasars, is controversial and over the lastdecade a bimodal distribution of radio loudness of quasars has been bothaffirmed and disputed. We model the quasar radio luminosity distribution withsimple unimodal and bimodal distribution functions. The resulting simulatedsamples are compared to a fiducial sample of 8,300 quasars drawn from the SDSSDR7 Quasar Catalog and combined with radio observations from the FIRST survey.Our results indicate that the SDSS-FIRST sample is best described by a radioloudness distribution which consists of two components, with 12+/-1 % ofsources in the radio-loud component. On the other hand, the evidence for alocal minimum in the loudness distribution (bimodality) is not strong and wefind that previous claims for its existence were probably affected by theincompleteness of the FIRST survey close to its faint limit. We alsoinvestigate the redshift and luminosity dependence of the radio loudnessdistribution and find tentative evidence that at high redshift radio-loudquasars were rarer, on average "louder", and exhibited a smaller range in radioloudness. In agreement with other recent work, we conclude that the SDSS-FIRSTsample strongly suggests that the radio loudness distribution of quasars is nota universal function, and that more complex models than presented here areneeded to fully explain available observations.

Disclosing the Radio Loudness Distribution Dichotomy in Quasars: An Unbiased Monte Carlo Approach Applied to the SDSS-FIRST Quasar Sample