Paper detail

FRII radio galaxies in the SDSS: Observational facts

Starting from the Cambridge Catalogues of radio sources, we have created a sample of 401 FRII radio sources that have counterparts in the main galaxy sample of the 7th Data release of the SDSS and analyse their radio and optical properties. We find that the luminosity in the Halpha line - which we argue gives a better measure of the total emission-line flux than the widely used O[III] luminosity - is strongly correlated with the radio luminosity P1.4GHz. We show that the absence of emission lines in about one third of our sample is likely due to a detection threshold and not to a lack of optical activity. We find that the properties of FRII galaxies are mainly driven by the Eddington parameter LHa/"MBH" or, equivalently, P1.4GHz/"MBH". Radio galaxies with hot spots are found among the ones with the highest values of P1.4GHz/"MBH". Compared to classical AGN hosts in the main galaxy sample of the SDSS, our FRII galaxies show a larger proportion of objects with very hard ionizing radiation field and large ionization parameter. A few objects are, on the contrary, ionized by a softer radiation field. We find that the black hole masses and stellar masses in FRII galaxies are very closely related. A comparison sample of line-less galaxies in the SDSS follows the same relation, although on average the masses are smaller. This suggests that the FRII radio phenomenon occurs in normal elliptical galaxies, preferentially in the most massive ones. Although most FRII galaxies are old, some contain traces of young stellar populations. Such young populations are not seen in normal line-less galaxies, suggesting that the activity in some FRII galaxies may be triggered by recent star formation. The "MBH"-Mgal relation in a comparison sample of radio-quiet AGNs from the SDSS is very different, suggesting that galaxies which are still forming stars are also still building their central black holes.