Heating of the IGM by FRII radio sources

Abstract

We present results of a numerical integration of the hydrodynamical equations governing the self-similar, two-dimensional gas flow behind the bow shock of an FRII radio source embedded in an IGM with a power law density profile. The model predicts pressure gradients within the cocoons consistent with modest backflow. For very steep external density profiles sources may well not expand in a self-similar fashion and in this case the model is not self-consistent. The assumption of ram pressure confinement of the cocoons perpendicular to the jet axis is found to overestimate the ratio of the pressure in front of the radio hot spots and that in the cocoons. Based on the properties of the gas between bow shock and cocoon we calculate the X-ray surface brightness of the flow. This emission is found to be a good tracer of the density distribution within the flow and varies significantly with the properties of the unshocked IGM. The cooling-time of the shocked IGM is found to be comparable to, or greater than, the Hubble time. The influence of a radio source on the evolution of its gaseous surroundings therefore extends well beyond the limited life time of the source itself. We compare our results with the X-ray map of Cygnus A and find some evidence for cold, dense gas clumps in the surroundings of this object. The extended X-ray emission observed around 3C 356 may also be caused by the bow shock of this radio source. We also present an empirical model for the X-ray emission of the shocked IGM due to thermal bremsstrahlung.