  We have recently shown that X-ray observations of the population of
`low-excitation' radio galaxies, which includes most low-power, Fanaroff-Riley
class I sources as well as some more powerful Fanaroff-Riley class II objects,
are consistent with a model in which the active nuclei of these objects are not
radiatively efficient at any waveband. In another recent paper Allen et al.
have shown that Bondi accretion of the hot, X-ray emitting phase of the
intergalactic medium (IGM) is sufficient to power the jets of several nearby,
low-power radio galaxies at the centres of clusters. In this paper we combine
these ideas and suggest that accretion of the hot phase of the IGM is
sufficient to power all low-excitation radio sources, while high-excitation
sources are powered by accretion of cold gas that is in general unrelated to
the hot IGM. This model explains a number of properties of the radio-loud
active galaxy population, and has important implications for the energy input
of radio-loud active galactic nuclei into the hot phase of the IGM: the energy
supply of powerful high-excitation sources does not have a direct connection to
the hot phase.
