Extragalactic radio sources with hybrid morphology: implications for the Fanaroff-Riley dichotomy

We provide observational and theoretical perspectives on the currently much debated issue of the Fanaroff-Riley (FR) morphological dichotomy of extragalactic radio sources. In this context we introduce a new, albeit rare, class of double radio sources in which the two lobes exhibit clearly different FR morphologies. It is argued that such `HYbrid MOrphology Radio Sources', or HYMORS, could be used to effectively constrain the theoretical mechanisms proposed for the FR dichotomy. Basically, the existence of HYMORS supports explanations for the FR dichotomy based upon jet interaction with the medium external to the central engine, and appears quite difficult to reconcile with the class of explanations that posit fundamental differences in the central engine, such as black hole spin or jet composition, to be responsible for the two FR classes of double radio sources.