The relationship of the Fanaroff-Riley classification of extragalactic radio sources to jet physics.

The Class I/Class II division of extragalactic radio sources by Fanaroff and Riley (1974) is a manifestation of important physical differences existing in radio sources. It is proposed that the division essentially arises from the differing Mach numbers in Class I and Class II jets. The low Mach number, Class I jets are susceptible to turbulence, are decelerated by entrainment of the surrounding medium, and maintain an anomalously high surface brightness as a result. The high Mach number, Class II jets are less turbulent and remain supersonic, produce high pressure shocks along their lengths and terminate via a strong shock against the IGM. An analysis of the energy balance in both types of source reveals jet velocities of the order of 5-10,000 km/s for Class I jets and mildy relativistic velocities for Class II jets. The important role of optical and X-ray observations in determining the gravitational field of pressure distribution in radio galaxies will be discussed with examples given on NGC 1399 and IC 4296.