8.2. The Second Dynamical Component
The unique aspect of Cygnus A is that it provides the opportunity to study the interaction between the radio source and the ICM in much greater detail than for any other powerful radio galaxy. Until now our understanding of the overall dynamics of powerful radio sources embedded in dense cluster atmospheres has been based strictly on radio observations. The advent of the sensitive, high resolution imaging capability of the ROSAT has finally revealed clearly the long hypothesized effects of the expanding radio source on the cluster gas.
The ROSAT HRI image of Cygnus A is shown in Fig. 10. This image reveals clearly the presence of the radio source within the cluster: X-ray emission from the hotspot regions, most of which is probably non-thermal and discussed below, deficits of X-ray surface brightness in the regions coincident with the inner parts of the radio lobes, and knots of excess X-ray emission along some of the edges of the radio source (see also Fig. 3 in Carilli et al. 1994b).
Figure 10. The grey-scale is a radio image of Cygnus A at 327 MHz with 5" resolution. The contours are from a ROSAT HRI observation of Cygnus A, reproduced from Carilli et al. (1994b), showing (predominantly) thermal X-ray emission from the inner regions of the Cygnus A cluster. Note that low surface brightness thermal X-ray emission continues to radii 10 x that shown on this image (Arnaud et al. 1984). The (presumably) non-thermal X-ray emission from the radio hotspot regions is also visible on this plot.
Clark and Harris present results from a 3D numerical simulation of the expected X-ray surface brightness distribution in the centers of clusters containing supersonically expanding radio sources such as Cygnus A. They show that the general anti-correlation between X-ray and radio surface brightness in Cygnus A within 35" of the nucleus is consistent with the jet model for powerful radio galaxies. The deficits correspond to regions evacuated by the radio lobes, while the excesses correspond to emission from gas which has been displaced by the radio source. From the depth of the `holes' in the X-ray gas at the positions of the radio lobes, they conclude that the radio source must effectively exclude the ICM, implying a contact discontinuity that is globally stable to mixing, and also that the transverse expansion of the shocked ICM must be substantial, otherwise the emission from the shocked gas would fill-in the observed holes. These conclusions are generally consistent with the analytic model of Begelman and Cioffi (1989) for shocked sheaths of ICM surrounding the lobes of powerful radio galaxies, in which they predict a width for the shocked ICM in Cygnus A 0.5 x length of the lobes.