Multifrequency VLA observations of PKS 0745-191: The archetypal 'cooling flow' radio source?

Abstract

We present 90-, 20-, 6-, and 2-cm VLA§ observations of the high radio luminosity, cooling flow radio source PKS 0745-191. We find that the radio source has a core with a very steep spectrum (a≃ -1) and diffuse emission with an even steeper spectrum (α 7sime; - 1.5 to - 2.3) without clear indications of the jets, hotspots or double lobes found in other radio sources of comparable luminosity. The appearance of the source is highly dependent on frequency and resolution. This dependence reflects both the diffuse nature of the extended emission and the steep, but position-dependent, spectrum of the radio emission. The cooling flow itself and processes associated with it are unable to provide sufficient energy to power the radio source. This includes energy input from shocks, supernovae, and magnetic reconnection in the inflowing, cooling gas. Thus, we favour an interpretation in which the energy input comes from the central engine, but in which the structure of the source may be heavily influenced by the past history of the galaxy and the inflowing intracluster medium. We show that, while the radio source is energetically unimportant for the cluster as a whole, it is certainly important on the scale of the cooling flow. The radio source should be an important source of local heating, in the form of kinetic energy, cosmic rays, and enhancement of thermal conductivity. The mere existence of cosmic rays and magnetic fields within a substantial fraction of the volume inside the cooling radius has important consequences for cooling-flow models. Since the radio data imply that large amounts of energy are being deposited in the cluster core, we suggest that non-steady state models for the cooling flow in PKS 0745-191 should be considered.