On black hole masses and radio loudness in active galactic nuclei

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 x 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.