Implications of the continuous radio-loudness distribution among AGNs in the local Universe

Aims. We investigate the radio loudness ($\mathcal{R}$) distribution in a large, homogeneous sample of radio galaxies. Methods. The sample is composed of galaxies from the ROGUE I/II catalogue belonging to the SDSS MGS and is divided into optically inactive radio galaxies (OPIRGs), optically active ones (OPARGs) and radio Seyferts. We use optical, mid-infrared and radio data to calculate the AGN bolometric luminosities, accretion rate ($\lambda$), black-hole mass ($M_{BH}$) and $\mathcal{R}$. 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 $\mathcal{R}$. The highest $\mathcal{R}$ values are associated with extended radio structures. $\mathcal{R}$ is anti-correlated with $\lambda$, and spans about 2 dex at fixed $\lambda$. Radio Seyferts, OPARGs and OPIRGs form a sequence of increasing $M_{BH}$ with substantial overlap. Radio Seyferts show no $\mathcal{R}$-$M_{BH}$ correlation, whereas OPARGs and OPIRGs show a weak positive trend. From theoretical considerations, the observed two-dex spread in radio luminosity and $\mathcal{R}$ can be reproduced by a four-fold variation in the dimensionless magnetic flux $\phi$ 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 $\phi$ may account for the observed scatter in this relation.