The expansion and cosmological evolution of powerful radio sources

Abstract

A simple analytical model is formulated to study beam dynamics in powerful extragalactic radio sources. Two phases are considered for the beam propagation: first through the gaseous halo of the parent galaxy and then through a much hotter intergalactic medium (IGM). These two media are conceived to be pressure-matched at their interface, which is expected to move closer to the parent galaxy with increasing redshift, owing to a steeply rising IGM pressure. We argue that the advance of the beam can be ignored quantitatively (notwithstanding a continuing nuclear activity) once its rate has become subsonic relative to the ambient medium. For the input parameters to the model, we adopt from the literature typical values for a powerful radio source, its associated X-ray emitting halo, and the IGM, and show that the model can explain in a fairly natural way the observed linear sizes of the sources at small redshifts and also their cosmological evolution. The possible relevance of the model to the reported prevalence of subgalacticsized sources at very high redshifts (z ≳ 2) is also discussed.