Fanaroff–Riley classification

The Fanaroff–Riley classification is a scheme created by B.L. Fanaroff and J.M. Riley in 1974, which is used to distinguish radio galaxies with active nuclei based on their radio luminosity or brightness of their radio emissions in relation to their hosting environment. Fanaroff and Riley noticed that the relative positions of high/low surface brightness regions in the lobes of extragalactic radio sources are correlated with their radio luminosity. Their conclusion was based on a set of 57 radio galaxies and quasars that were clearly resolved at 1.4 GHz or 5 GHz into two or more components. Fanaroff and Riley divided this sample into two classes using the ratio of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar to the total extent of the source up to the lowest brightness contour. Class I (abbreviated FR-I) are sources whose luminosity decreases as the distance from the central galaxy or quasar host increase, while Class II (FR-II) sources exhibit increasing luminosity in the lobes. This distinction is important because it presents a direct link between the galaxy's luminosity and the way in which energy is transported from the central region and converted to radio emission in the outer parts.