Implications of the continuous radio-loudness distribution among active galactic nuclei in the local Universe

Aims. We investigate the radio loudness (ℛ) distribution in a large, homogeneous sample of radio galaxies. Methods. The sample is composed of galaxies from the ROGUE I/II catalogue that belongs to the SDSS main galaxy sample and is divided into optically inactive radio galaxies (OPIRGs), optically active ones (OPARGs), and ’radio Seyferts’. We used optical, mid-infrared, and radio data to calculate the active galactic nucleus bolometric luminosities, accretion rate ( λ ), black-hole mass ( M BH ), and ℛ. Results. Contrary to some previous studies based on restricted samples, using our complete sample of objects with redshifts z < 0.4, we find no evidence of bimodality in ℛ. The highest ℛ values are associated with extended radio structures. We find that ℛ is anti-correlated with λ , and spans ∼2 dex at fixed λ . Radio Seyferts, OPARGs, and OPIRGs form a sequence of increasing M BH with substantial overlap. Radio Seyferts show no correlation ℛ– M BH , whereas OPARGs and OPIRGs show a weak positive trend. From theoretical considerations, the observed ∼2-dex spread in radio luminosity and ℛ can be reproduced by only an approximately four-fold variation in the dimensionless magnetic flux φ assuming realistic black-hole spins. Conclusions. The smooth distribution of radio loudness supports a common evolutionary path for all radio sources, with black-hole spin and magnetic field varying continuously. The radio loudness depends on black-hole mass and accretion rate, while moderate variations in φ may account for the observed scatter in this relation.