2011MNRAS.416..917C

We investigate the relationship between the mass of central supermassive black holes (SMBHs) and the radio loudness of active galactic nuclei (AGNs). We use the most recent calibrations to derive `virial' BH masses for samples of radio-loud quasi-stellar objects (QSOs) for which relatively small masses (M<sub>BH</sub>< 10<sup>8</sup> M<sub>☉</sub>) have been estimated in the literature. We take into account the effect of radiation pressure on the broad-line region (BLR) which reduces the `effective' gravitational potential experienced by the broad-line clouds and affects the mass estimates of bright quasars. We show that in well-defined samples of nearby low-luminosity AGNs (LLAGNs), QSOs and AGNs from the Sloan Digital Sky Survey, radio-loud AGNs invariably host SMBHs exceeding ∼10⁸ M_☉. On the other hand, radio-quiet AGNs are associated with a much larger range of BH masses. The overall result still holds even without correcting the BH mass estimates for the effects of radiation pressure. We present a conjecture based on these results, which aims at explaining the origin of radio loudness in terms of two fundamental parameters: the spin of the BH and the BH mass. We speculate that in order to produce a radio-loud AGN, both of the following requirements must be satisfied: (1) the BH mass, M_BH, has to be larger than ∼10⁸ M_☉, and (2) the spin of the BH must be significant, in order to satisfy theoretical requirements. Taking into account the most recent observations, we envisage a scenario in which the merger history of the host galaxy plays a fundamental role in accounting for both the properties of the AGN and the galaxy morphology, which in our picture are strictly linked. On the one hand, radio-loud sources might be obtained only through major `dry' mergers involving BH of large mass, which would give rise to both the `core' morphology and the significant BH spin needed. On the other hand, radio-quiet AGNs might reside in galaxies that underwent different evolutionary paths, depending on their BH mass.