The dividing line between FR I and FR II radio–galaxies

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 ∼ 10–10 of the Eddington one. This possibly regulates the accretion mode and the consequent presence of nuclear outflows.