On the origin of Fanaroff-Riley classification of radio galaxies: Deceleration of supersonic radio lobes

TL;DR

This paper explains the Fanaroff-Riley classification of radio galaxies by analyzing how the deceleration of radio lobes, driven by growth in their cross-sectional area, determines whether lobes are subsonic or supersonic, based on jet power and ambient density.

Contribution

It introduces a model linking the FRI/FRII dichotomy to the deceleration of radio lobes caused by their increasing cross-sectional area, deriving a critical ratio of jet power to ambient density.

Findings

The FRI/FRII division is sharp in the radio-optical luminosity plane.

The transition from supersonic to subsonic lobes depends on the ratio of jet power to ambient density.

A maximal ratio of jet power to ambient density separates FRI and FRII sources.

Abstract

We argue that the origin of "FRI/FRI{-.1em}I dichotomy" -- the division between Fanaroff-Riley class I (FRI) with subsonic lobes and class I{-.1em}I (FRI{-.1em}I) radio sources with supersonic lobes is sharp in the radio-optical luminosity plane (Owen-White diagram) -- can be explained by the deceleration of advancing radio lobes. The deceleration is caused by the growth of the effective cross-sectional area of radio lobes. We derive the condition in which an initially supersonic lobe turns into a subsonic lobe, combining the ram-pressure equilibrium between the hot spots and the ambient medium with the relation between "the hot spot radius" and "the linear size of radio sources" obtained from the radio observations. We find that the dividing line between the supersonic lobes and subsonic ones is determined by the ratio of the jet power $L_{\rm j}$ to the number density of the ambient matter at the core radius of the host galaxy $\bar{n}_{\rm a}$. It is also found that there exists the maximal ratio of $(L_{\rm j}/\bar{n}_{\rm a})$ and its value resides in $(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}\approx 10^{44-47} {\rm erg} {\rm s}^{-1} {\rm cm}^{3}$, taking account of considerable uncertainties. This suggests that the maximal value $(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}$ separates between FRIs and FRI{-.1em}Is.