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

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/radioluminosity 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.

The morphological classification of distant radio galaxies explored with three-dimensional simulations

“…This is illustrated in the right panel of Fig 18. The turbulent dissipation of energy within low Mach number jets was postulated by Bicknell (1985) and is clearly relevant to the radio structure of lowpowered radio galaxies. However, it is also established that FR I jets are intrinsically different in composition: -it is not just a matter of Mach number (Croston et al 2018). In addition, the jet speed itself is a potential factor as discussed fro the compact FR 0 sources by Baldi et al (2019), It is conjectured that mechanical feedback from active galactic nuclei is a key process in the evolution of galaxy clusters (e.g.…”

The Massive and Distant Clusters of WISE Survey. VII. The Environments and Properties of Radio Galaxies in Clusters at z ∼ 1

“…Additionally, FR II sources are generally more luminous than FR I sources (Fanaroff & Riley 1974;Ledlow & Owen 1996), though this distinction is not absolute . And lastly, it has been shown that the internal composition radio-lobe plasma is systematically different in these two types of radio galaxies (Croston et al 2018); the particle content of FRI lobes is dominated by protons and ions, whereas FRIIs are primarily composed of an electron-positron plasma.…”

Revisiting the Fanaroff–Riley dichotomy and radio-galaxy morphology with the LOFAR Two-Metre Sky Survey (LoTSS)

“…An additional complication in using radio observational data to test physical models for jet dynamics and the FR divide is the weak relationship between jet power and radio luminosity. In particular, a systematic difference in the efficiency of producing radio luminosity for a given jet power for FRIs and FRIIs is thought to exist (Croston et al 2018), caused by the correlation of FR class with lobe particle content. FRI radio galaxies are found to be energetically dominated by heavy particles (protons and ions), while FRII radio galaxies are primarily composed of an electron-positron plasma (Croston et al 2018) -this situation may be best explained by the role of entrainment of surrounding material into disrupted FRI jets as they decelerate, while undisrupted FRII jets remain more "pristine".…”