Optically active and optically inactive radio galaxies as sub-populations of the main galaxy sample of the SDSS

Aims. We use the ROGUE I and II catalogues of radio sources associated with optical galaxies to revisit the characterization of radio active galactic nuclei (AGNs) in terms of radio luminosities and properties derived from the analyses of the optical spectra of their associated galaxies. Methods. We propose a physically based classification of radio galaxies into ‘optically inactive’ and ‘optically active’ (OPARGs and OPIRGs). In our sample, there are 14 082 OPIRGs and 2721 OPARGs. After correcting for the Malmquist bias, we compared the global properties of our two classes of radio galaxies and put them in the context of the global population of galaxies. To compare the Eddington ratios of OPARGs with those of Seyferts, we devised a method to obtain the bolometric luminosities of these objects, taking into account the contribution of young stars to the observed line emission. We provide formulae to derive bolometric luminosities from the [O III ] luminosity. Results. We find that the distributions of radio luminosities of OPARGs and OPIRGs are undistinguishable. On average, the black hole masses and stellar masses in OPIRGs are larger than in OPARGs. OPARGs show signs of some recent star formation. Plotting the OPARGs in the BPT diagram and comparing their distribution with that of the remaining galaxies, we find that there is a sub-family of very high excitation OPARGs at the top of the AGN wing. This group is slightly displaced towards the left of the rest of the AGN galaxies, suggesting a stronger ionizing radiation field with respect to the gas pressure. Conclusions. Only very-high excitation radio galaxies (VHERGs) have Eddington ratios higher than 10 −2 , which are canonically considered as the lower limit for the occurrence of radiative efficient accretion. If our estimates of the bolometric luminosities are correct, this means than only a small proportion of mainstream HERGs are indeed radiatively efficient.