The Fanaroff-Riley transition and the optical luminosity of the host elliptical galaxy

We show that a model for radio source dynamics we had earlier proposed can readily reproduce the relationship between the radio power division separat ing the two Fanaroff-Riley classes of extragalactic radio sources and the o ptical luminosity of the host galaxy, as found by Owen & Ledlow (1994). In our scenario, when less powerful jets eventually slow down to the point tha t the advance of the working surface (i.e., hotspot) becomes subsonic with respect to the external gas, the jet's collimation is severely weakened. Th is criterion distinguishes the powerful and well collimated FR II sources f rom the weaker sources producing the less collimated FR I type morphologies .