On the origin of radio-loudness in AGNs and its relationship with the
  properties of the central supermassive black hole
  We investigate the relationship between the mass of central supermassive
black holes and the radio loudness of active galactic nuclei. We use the most
recent calibrations to derive virial black hole masses for samples of radio
loud QSOs for which relatively small masses (M_BH<10^8 M_sun) have been
estimated in the literature. We take into account the effect of radiation
pressure on the 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,
QSOs and AGNs from the SDSS, radio-loud (RL) AGN invariably host SMBHs
exceeding ~10^8 M_sun. On the other hand, radio-quiet (RQ) AGNs are associated
with a much larger range of black hole 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 black hole and the black hole mass. We speculate that in order
to produce a RL AGN both of the following requirements must be satisfied: 1)the
black hole mass M_BH has to be larger than ~10^8 M_sun; 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. RL 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 black hole spin needed.
