In the radio – host galaxy optical luminosity plane FR I and FR II radio–galaxies are clearly divided .
Since the optical luminosity of an elliptical galaxy is an indication of the mass of its central black hole , we propose that the FR I–FR II dividing luminosity is a function of the mass of the black hole powering the active nucleus .
Furthermore , as the radio power gives an estimate of the total kinetic power carried by the jet , the FR I–FR II separation can be re–interpreted as occurring at a constant ratio between the jet power and the black hole mass .
There is also convincing evidence of a correlation between the radio power and the luminosity in narrow emission lines .
As the latter results from photoionization by the radiation produced by accretion , we can estimate the ionizing luminosity and find that the separation luminosity can be also re–expressed as a constant accretion rate between \sim 10 ^ { -2 } – 10 ^ { -3 } of the Eddington one .
This possibly regulates the accretion mode and the consequent presence and characteristics of nuclear outflows .