Cosmic evolution of linear sizes and luminosity function of powerful radio galaxies - is there a common cause?

Abstract

For the population of active elliptical galaxies associated with powerful double radio sources, it is proposed that the empirically derived brightening of their radio luminosity function (RLF) and the statistical decrease in their linear size at higher redshifts (up to z ~ 1, or 2) could both arise if the hot gaseous halos seen to surround the ellipticals were denser at earlier epochs. The postulated density increase of the halo medium by about an order-of-magnitude between z = 0 and 1 would not just impart an extra stopping power needed to explain the smaller sizes of the double radio sources in the past but would, at the same time, enhance their efficiency for converting the beam power into radio emission by a factor which is quantitatively shown to be consistent with the empirically deduced evolution of the RLF up to z ~ 1. It seems thus possible to envision a unified framework for understanding the physical origin of these two well-known cosmological trends displayed by powerful radio galaxies. We also briefly explore a possible physical link between the two basic morphological classes of extragalactic radio sources, namely the powerful, edge brightened and the weaker, edge-darkened double radio sources.