The onset of a powerful radio source in a central cluster galaxy

Abstract

We discuss the effects of the onset of a powerful Fanaroff-Riley (FR) type II radio source in a central cluster galaxy on the surrounding cluster gas. We show that many observed properties of powerful high-redshift radio sources can be accounted for if the sources are at the centres of clusters with strong cooling flows. The cooling flow provides a ready source of cool and cold gas which can be the source of optical emission- and absorption-line clouds in both quasars and galaxies, along with a means of hiding the central engine in some sources. The scenario predicts correlations between radio source size and the presence (or lack) of associated optical absorption and the strength of any aligned optical light. In particular, strong Lyα absorption should be common in radio galaxies; the smaller the region of radio emission, the stronger the absorption. If a large fraction of the associated absorption seen in quasars arises from surrounding cooling flows, less luminous quasars should show considerably more associated absorption. We show that a cluster environment for powerful radio sources is compatible with the interaction picture for the triggering of radio activity in these sources. Finally, we note that if powerful radio sources are at the centres of clusters at high redshift, then they are markers for systems with hundreds or thousands of times the mass of an ordinary galaxy. Consequently, evolution in their comoving number density severely constrains hierarchical clustering cosmologies.