A new connection between the opening angle and the large-scale morphology of extragalactic radio sources

TL;DR

This study links the opening angle of conical jets to the large-scale morphology of extragalactic radio sources, using simulations to identify key scales that influence whether sources appear as FR I or FR II types.

Contribution

It introduces a model connecting jet opening angle and external pressure to the large-scale morphology of radio sources, supported by hydrodynamic simulations.

Findings

The relative scales determine jet morphology and Mach number.

FR I sources likely have large intrinsic opening angles.

All FR I sources with lobes probably evolved from FR II phase.

Abstract

In the case of an initially conical jet, we study the relation between jet collimation by the external pressure and large-scale morphology. We first consider the important length-scales in the problem, and then carry out axisymmetric hydrodynamic simulations that include, for certain parameters, all these length-scales. We find three important scales related to the collimation region: (i) where the sideways ram-pressure equals the external pressure, (ii) where the jet density equals the ambient density, and (iii) where the forward ram-pressure falls below the ambient pressure. These scales are set by the external Mach-number and opening angle of the jet. We demonstrate that the relative magnitudes of these scales determine the collimation, Mach-number, density and morphology of the large scale jet. Based on analysis of the shock structure, we reproduce successfully the morphology of Fanaroff-Riley (FR) class I and II radio sources. Within the framework of the model, an FR I radio source must have a large intrinsic opening angle. Entrainment of ambient gas might also be important. We also show that all FR I sources with radio lobes or similar features must have had an earlier FR II phase.