The nuclear radio – optical properties of intermediate redshift FR II radio galaxies and quasars ⋆

We extend the study of the nuclei of 3CR objects as seen with the HST to higher redshift FR II radio sources (0.4 < z < 0.6). Our results reflect what has been found for FR II of lower redshift. The position of the nuclei on the plane formed by optical and radio luminosities is related to their optical spectral classification: the nuclei of both high and low excitation galaxies lie on the correlation found for FR I sources, while broad–lined objects have a significant optical excess. The nuclear properties of these sources are better understood when considering the equivalent width of the [OIII] emission line with respect to their optical luminosities. Even in the range of redshift considered here, low excitation galaxies show peculiar nuclear properties, more similar to those observed in FR I. This confirms that not all FR II are unified with quasars. Our findings have important implications for the FR II–quasar unification scheme: by reconsidering the classification of all 3CR radio galaxies with z < 1 in the light of their nuclear properties, we derive the opening angle of the obscuring torus for different redshift bins. We find that the covering factor of the obscuring structure decreases as redshift increases (θ ∼ 44 • for z < 0.3 to θ ∼ 56 • for 0.5 < z < 1). We argue that this behavior may be interpreted in the framework of the receding torus model, in which the opening angle of the torus increases as the luminosity of the accretion disk around the central black hole increases.