FR II radio galaxies in the Sloan Digital Sky Survey: observational facts

Starting from the Cambridge Catalogues of radio sources, we have created a sample of 401 Fanaroff-Riley type II (FR II) radio sources that have counterparts in the main galaxy sample of the seventh Data release of the Sloan Digital Sky Survey (SDSS) and analyse their radio and optical properties.

We find that the luminosity in the Hα line - which we argue gives a better measure of the total emission-line flux than the widely used luminosity in [O III]- is strongly correlated with the radio luminosity P_{1.4 GHz}. 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 also find a very strong correlation between the values of L_Hα and P_{1.4 GHz} when scaled by ‘M_BH’, an estimate of the black hole mass.

We find that the properties of FR II galaxies are mainly driven by the Eddington parameter L_Hα/‘M_BH’ or, equivalently, P_{1.4 GHz}/‘M_BH’. Radio galaxies with hotspots are found among the ones with the highest values of P_{1.4 GHz}/‘M_BH’.

Compared to classical active galactic nuclei (AGN) hosts in the main galaxy sample of the SDSS, our FR II 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. Two of them have double-peaked emission lines and deserve more attention.

We find that the black hole masses and stellar masses in FR II galaxies are very closely related: ‘M_BH’∝M^1.13_* with very little scatter. A comparison sample of line-less galaxies in the SDSS follows exactly the same relation, although the masses are, on average, smaller. This suggests that the FR II radio phenomenon occurs in normal elliptical galaxies, preferentially in the most massive ones. Although most FR II galaxies are old, some contain traces of young stellar populations. Such young populations are not seen in normal line-less galaxies, suggesting that the radio (and optical) activity in some FR II galaxies may be triggered by recent star formation. The ‘M_BH’-M_* relation in a comparison sample of radio-quiet AGN hosts from the SDSS is very different, suggesting that galaxies which are still forming stars are also still building their central black holes.

Globally, our study indicates that, while radio and optical activity are strongly related in FR II galaxies, the features of the optical activity in FR IIs are distinct from those of the bulk of radio-quiet active galaxies.

An appendix (available as Supporting Information with the online version of the article) gives the radio maps of our FR II galaxies, superimposed on the SDSS images, and the parameters derived for our analysis that were not publicly available.