Radio Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications

We investigate how the radio-loudness of Active Galactic Nuclei (AGN) depends on their bolometric luminosity expressed in Eddington units (Eddington ratio) and on the central black-hole mass. In our sample we include the, often missed in other similar studies, broad-line-radio-galaxies (BLRG) and find that for the same range of Eddington ratios, the radio-loudness of BLRG is on average 3 orders larger than the radio-loudness of Seyfert galaxies. Both sub-samples form sequences along which the radio-loudness decreases with increasing Eddington ratio with slopes corresponding roughly to constant radio luminosity. At very low luminosities these two AGN sequences match respectively the sequences of FRI radio-galaxies and LINERS, but with an indication of a radio-loudness saturation. We show that these features can be naturally reproduced by assuming that the normalization of the dependence of radio-loudness on the Eddington ratio is determined by the black-hole spin and that central black holes in giant elliptical galaxies have on average spins much larger than black holes in the centers of spiral/disc galaxies. Finally, we address the issue of radio-loudness dichotomy among the near-Eddington AGN, and suggest that at high accretion rates jet production, and therefore radio-loudness, might be suppressed as is directly observed in several X-ray binaries.