We have investigated the influence of nuclear parameters such as black hole mass and photoionizing luminosity on the FRI/FRII transition in a sample of nearby ( z < 0.2 ) radio galaxies from the 3CR catalogue .
The sample was observed with medium-resolution , optical spectroscopy and contains some galaxies with unpublished velocity dispersion measurements and emission-line fluxes .
The measured velocity dispersions for the sample lie in the range 130–340 km s ^ { -1 } with a mean of 216 km s ^ { -1 } .
Using the M- \sigma relation , we convert to black hole mass and find that the black hole mass distribution is identical for FRI and FRII galaxies , with a mean of \approx 2.5 \times 10 ^ { 8 } M _ { \odot } .
We determine narrow emission-line luminosities from [ O ii ] and [ O iii ] in our spectra , as well as from the literature , and convert them to photoionizing luminosities under the assumption that the gas is ionized by the nuclear UV continuum .
Most of the galaxies with FRI morphology and/or low-excitation emission-line spectra have progressively lower black hole masses at lower photoionizing ( and jet ) luminosities .
This agrees with the well-known Ledlow-Owen relation which states that the radio luminosity at the FRI/FRII transition depends on the optical luminosity of the host , L _ { radio } \propto L _ { optical } ^ { 1.8 } , because these two luminosities relate to AGN nuclear parameters .
When recasting the Ledlow-Owen relation into black hole mass versus photoionizing luminosity and jet luminosity , we find that the recasted relation describes the sample quite well .
Furthermore , the FRI/FRII transition is seen to occur at approximately an order of magnitude lower luminosity relative to the Eddington luminosity than the soft-to-hard transition in X-ray binaries .
This difference is consistent with the Ledlow-Owen relation , which predicts a weak black hole mass dependence in the transition luminosity in Eddington units .
We conclude that the FRI/FRII dichotomy is caused by a combination of external and nuclear factors , with the latter dominating .