With the current generation of X-ray observatories, Chandra and XMM–Newton, it has become possible to test the equipartition assumption rigorously, and to understand in what circumstances it applies. Even if non-radiating particles are accounted for, there is no firm theoretical basis for expecting equipartition to apply across the radio-galaxy population. relativistic protons, or material entrained from the surrounding medium), then equipartition calculations may underestimate the total energy by a large factor. However, if radio-galaxy jets and lobes contain non-radiating particles (e.g. The minimum energy assumption is closely equivalent to assuming the equipartition of energy between relativistic leptons and magnetic field, and such approaches have been widely used to estimate magnetic field strengths and radio source energetics. Burbidge ( 1956) made the first estimates of the energy content of a radio galaxy by assuming the minimum total energy that could produce the observed radio synchrotron emission. Galaxies: active, X-rays: galaxies: clusters 1 INTRODUCTIONĭetermining the particle content of radio galaxies has been a long-standing challenge, as the radio synchrotron emission with which they are primarily observed does not uniquely determine the source internal energy. We conclude that inferences of population-wide AGN impact require careful assessment of the contribution of different jet subclasses, 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. 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 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. This difference is best explained by the presence of an energetically dominant proton population in the FRI, but not the FRII radio galaxies. 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. A crucial and long-standing problem is the composition of the radio-lobe plasma that traces AGN jet activity. Ongoing and future radio surveys aim to trace the evolution of black hole growth and feedback from active galactic nuclei (AGNs) 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.
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