Gaseous halos of elliptical galaxies, the cosmic evolution of their radio sizes, and the phenomenon of compact steep-spectrum sources

Abstract

The observed dependences of the linear size of powerful radio galaxies on redshift and radio luminosity are investigated taking into account the likely variation of the "efficiency," ɛ, with which beam power is converted into radio emission. Our analytical model is based upon an assumed modest cosmological increase in the density of the halos through which the relativistic beams propagate. The enhancement of radio efficiency resulting from the increased density of the ISM into which the lobes are inflated by the beams is quantitatively estimated. We show that roughly half of the claimed linear size evolution can be understood in terms of this rise in efficiency, and thus the intrinsic evolution is considerably slower. By incorporating the dependence of ɛ on beam power, an excellent fit to the observed linear size-radio luminosity relation is obtained. This picture also provides a natural explanation for key properties of compact steep-spectrum sources; in particular, the observed steep rise in their abundance by z ~ 0.3 is probably caused by an amplification bias operating on intrinsically weaker beams propagating in a denser ISM caused by more frequent galaxy interactions in the past.