The HST view of the FR I FR II dichotomy

In order to explore how the FR I/FR II dichotomy is related to the nuclear properties of radio galaxies, we studied a complete sample of 26 nearby FR II radio galaxies using Hubble Space Telescope (HST) images and compared th em with a sample of FR I previously analyzed. FR I nuclei lie in the radio- optical luminosity plane along a tight linear correlation, which argues for a common synchrotron origin. FR II show a more complex behavior, which is however clearly related to their optical spectral classification. Broad line FR II radio galaxies (BLRG) are located overall well above the F R I correlation, suggesting that a contribution from thermal (disc) emissio n is present. Three narrow line (NLRG) and one weak line radio galaxy (WLRG ), in which no nuclear source is seen, can be interpreted as the obscured c ounter-parts of BLRG, in agreement with the current unification schemes. Conversely, in 5 sources of the sample, all of them NLRG or WLRG, optical c ores are located on the same correlation defined by FR I and with similar r adio and optical luminosities. This suggests that, in analogy to FR I, the emission is dominated by synchrotron radiation and represents the optical c ounter-part of the non-thermal radio cores. Interestingly, all these galaxi es are located in clusters, an environment typical of FR I. These results imply that, at least at low redshifts, the FR II population i s not homogeneous. Furthermore, the traditional dichotomy between edge dark ened and brightened radio morphology is not unequivocally connected with th e innermost nuclear structure, as we find FR II with FR I-like nuclei and t his has interesting bearings from the point of view of the AGN unified mode ls.