5.1. The Bridge and Plumes
At low frequency Cygnus A displays a `radio bridge' connecting the high surface brightness regions at the source extremities (Fig. 10). Whether the radio bridge forms a physically connected structure through the center of the galaxy, or just appears to in projection, remains an open question. The comparison of radio and line emission in the center of the Cygnus A galaxy (Jackson et al. 1994, Carilli et al. 1989b) suggests that the lobes may not have completely evacuated the inner 10 kpc of the galaxy.
At the center of the bridge are seen `plumes' of emission extending to the north and south. Leahy and Williams (1984) and Williams (1985) present a detailed analysis of radio bridges and bridges distortions in powerful radio galaxies, including Cygnus A. The plumes in Cygnus A are most consistent with Williams' model A in which back-flow of radio emitting fluid in the lobe is deflected away from the high pressure central regions of the galaxy by the general pressure gradient in the ICM.