RELATIVISTIC JETS AND THE FANAROFF-RILEY CLASSIFICATION OF RADIO GALAXIES

Recently, Owen and Owen & Ledlow have shown that the dividing line between Fanaroff-Riley class I and class II radio sources is very sharp when the sources are represented as points in the radio-optical luminosity plane. A theoretical expression for the dividing line is derived on the basis that borderline class I/II sources are characterized by (1) transonic Mach numbers and (2) more or less unique (approximate to 0.6c) mildly relativistic velocities, consistent with deceleration of an initially moderately relativistic or ultrarelativistic jet. The deduction of a unique transonic velocity is combined with established empirical relationships between the X-ray luminosities, core radii, velocity dispersions, and absolute magnitudes of elliptical galaxies. The slope of the theoretical dividing line is well determined and agrees well with that inferred from the data. The intercept of the dividing line depends upon several parameters which are known perhaps to within factors of order unity and agrees with the data to better than an order of magnitude. As a corollary to this work, useful relationships between the source total and monochromatic luminosities and the jet energy flux are obtained.