Double-double radio galaxies: further insights into the formation of the radio structures

TL;DR

This study models the dynamical evolution of double-double radio galaxies, revealing that their outer lobes fit standard models while inner lobes result from re-accelerated electrons, suggesting DDRGs are a typical phase in radio galaxy evolution.

Contribution

The paper provides a new interpretation of inner lobes in DDRGs as re-accelerated electrons, extending the standard model to explain their formation and evolution.

Findings

Outer lobes fit the Fanaroff-Riley II model

Inner lobes are caused by re-accelerated electrons within outer lobes

DDRGs may evolve into standard FR II radio galaxies

Abstract

Double-double radio galaxies (DDRGs) offer a unique opportunity for us to study multiple episodes of jet activity in large-scale radio sources. We use radio data from the Very Large Array and the literature to model two DDRGs, B1450+333 and B1834+620, in terms of their dynamical evolution. We find that the standard Fanaroff-Riley II model is able to explain the properties of the two outer lobes of each source, whereby the lobes are formed by ram-pressure balance of a shock at the end of the jet with the surrounding medium. The inner pairs of lobes, however, are not well-described by the standard model. Instead we interpret the inner lobes as arising from the emission of relativistic electrons within the outer lobes, which are compressed and re-accelerated by the bow-shock in front of the restarted jets and within the outer lobes. The predicted rapid progression of the inner lobes through the outer lobes requires the eventual development of a hotspot at the edge of the outer lobe, causing the DDRG ultimately to resemble a standard Fanaroff-Riley II radio galaxy. This may suggest that DDRGs are a brief, yet normal, phase of the evolution of large-scale radio galaxies.