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