Understanding the nature of FR II optical nuclei: a new diagnostic plane for radio galaxies⋆

We extend our study of the nuclei of 3CR FR II radio galaxies through HST optical images up to z = 0.3. In the majority of them an unresolved nucleus (central compact core, CCC) is found. We analyze their position in the plane formed by the radio and optical nuclear luminosities in relation to their optical spectral properties. The broad–lined objects (BLO) have the brightest nuclei: they are present only at optical luminosities νLν > ∼ 4 × 10 42 erg s which we suggest might represent a threshold in the radiative efficiency combined to a small range of black hole masses. About 40% of the high and low excitation galaxies (HEG and LEG) show CCC which resemble those previously detected in FR I galaxies, in apparent contrast to the unification model. The equivalent width of the [OIII] emission line (with respect to the nuclear luminosity) reveals the nature of these nuclei, indicating that the nuclei of HEG are obscured to our line of sight and only scattered radiation is observed. This implies that the population of FR II is composed by objects with different nuclear properties, and only a fraction of them can be unified with quasars.