On the relation between Fanaroff-Riley types I and II radio galaxies

Recent optical observations of radio galaxies reveal a transition from F-R II morphology to F-R I morphology with increasing optical luminosity at fixed radio luminosity. This transition can occur if a highly supersonic jet which normally fuels F-R II sources is instead strongly decelerated near the nucleus, producing the transonic or subsonic flow which gives rise to the F-R I morphology at the same radio power. It is shown that this deceleration can occur if the more optically luminous galaxies possess a dense inner ISM core, with an average density of order 10 cm^-3^ and a radius of order 1 kpc. The turbulent boundary layer of the high Reynolds number jet passing through this core entrains the ISM and imparts momentum to it, causing the jet to decelerate. A search of available photometry for these galaxies shows no significant color differences in B - V between the F-R I and F-R II objects, but a larger sample with emphasis on photometry of the nuclear region is required before any definitive conclusions can be drawn about differences in color.