2000ApJ...543L.111L

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>10⁹ M_☉ are radio-loud, while quasars with M_BH<3x10⁸ 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.