On the Origin of Radio Loudness in Active Galactic Nuclei
We investigate the relationship between the mass of central supermassive black holes (SMBHs) and the radio loudness of active galactic nuclei (AGNs). 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. 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. 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.