Particle content, radio-galaxy morphology and jet power: all radio-loud AGN are not equal

TL;DR

This study reveals that radio-loud AGN, specifically FRI and FRII types, differ systematically in plasma composition and jet power relationships, impacting how their feedback effects are interpreted across cosmic time.

Contribution

It provides the first conclusive comparison of plasma conditions in FRI and FRII radio galaxies, highlighting differences in particle content and their influence on jet power and radio luminosity relations.

Findings

FRI and FRII radio galaxies have systematically different plasma compositions.

FRI galaxies contain an energetically dominant proton population, unlike FRIIs.

Radio morphology influences the jet power to radio luminosity ratio.

Abstract

Ongoing and future radio surveys aim to trace the evolution of black hole growth and feedback from active galactic nuclei (AGN) throughout cosmic time; however, there remain major uncertainties in translating radio luminosity functions into a reliable assessment of the energy input as a function of galaxy and/or dark matter halo mass. A crucial and long-standing problem is the composition of the radio-lobe plasma that traces AGN jet activity. In this paper, we carry out a systematic comparison of the plasma conditions in Fanaroff & Riley class I and II radio galaxies to demonstrate conclusively that their internal composition is systematically different. This difference is best explained by the presence of an energetically dominant proton population in the FRI, but not the FRII radio galaxies. We show that, as expected from this systematic difference in particle content, radio morphology also affects the jet-power/radio-luminosity relationship, with FRII radio galaxies having a significantly lower ratio of jet power to radio luminosity than the FRI cluster radio sources used to derive jet-power scaling relations via X-ray cavity measurements. Finally we also demonstrate conclusively that lobe composition is unconnected to accretion mode (optical excitation class): the internal conditions of low- and high-excitation FRII radio lobes are indistinguishable. We conclude that inferences of population-wide AGN impact require careful assessment of the contribution of different jet sub-classes, particularly given the increased diversity of jet evolutionary states expected to be present in deep, low-frequency radio surveys such as the LOFAR Two-Metre Sky Survey.