Particle Acceleration and Diffusion in Fossil Radio Plasma

The strong activity of radio galaxies should have led to a nearly ubiquitous presence of fossil radio plasma in the denser regions of the inter-galactic medium as clusters, groups and filaments of galaxies. This fossil radio plasma can contain large quantities of relativistic particles (electrons and possibly protons) by magnetic confinement. These particles might be released and/or re-energized under environmental influences as turbulence and shock waves. Possible connections of such processes to the formation of the observed sources of diffuse radio emission in clusters of galaxies (the cluster radio halos and the cluster radio relics) are discussed. 1. The Cosmological Energy Budget of Radio Plasma The remnant of the radio lobes of a former active radio galaxy get rapidly invisible for radio telescopes, justifying the name ’radio ghosts’ for such patches of fossil radio plasma (Enßlin, 1999a). Radio ghosts should be an important ingredient of the inter galactic medium (IGM). The present space density of one or a few radio galaxies per galaxy cluster was exceeded by orders of magnitude during the epoch of violent quasar activity around z = 2. There are several routes to estimate the energy released by earlier radio galaxies. One is to integrate the evolution of the radio-luminosity-function, translated to jet-power via an empirical jet-power–radio-luminosity relation, based on equipartition energy densities of radio lobes. The result is (Enßlin & Kaiser, 2000) Ejet = 3 · 10 66 ergGpc (feq/3) , (1) where feq gives the ratio of true to equipartition energy density in the radio plasma, which is likely bigger than one. This amount of energy corresponds to ∼ 0.1 keV per cosmic baryon, and is therefore not negligible, since large-scale structure formation has produced ∼ 1 keV/baryon by gravitational infall. An independent route is to use the Magorrian relation (ηbh = Mbh/Mbulge = 0.002) between the galaxy bulge (Mbulge) and central black hole (Mbh) mass. Assuming an efficiency of εjet = 0.1 of rest mass to jet energy conversion of the accretion process feeding the black hole growth, one derives from the observed present day galaxy population (Enßlin et al., 1998b) Ejet = 3 · 10 67 ergGpc (ηbh/0.002) (εjet/0.1) . (2)