The distribution of radio to optical fluxes in active galactic nuclei (AGNs) is bimodal. The physical origin for this bimodality is not understood. In this Letter I describe observational evidence, based on the Boroson & Green Palomar-Green quasar sample, that the radio loudness bimodality is strongly related to the black hole mass ( M BH ). Nearly all PG quasars with M BH > 10 9 M ☉ are radio-loud, while quasars with M BH < 3 × 10 8 M ☉ are practically all radio-quiet. This result is consistent with the dependence of quasar host galaxy morphology on radio loudness. There is no simple physical explanation for this result, but it may provide a clue as to how jets are formed near massive black holes. The radio loudness-black hole mass relationship suggests that the properties of various types of AGN may be largely set by three basic parameters: M BH , L / L Edd , and inclination angle.
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A 2000 study studied this question.