The giant radio galaxy 8C 0821+695 and its environment

We present new VLA and Effelsberg observations of the radio galaxy 8C 0821695. We have obtained detailed images in total intensity and polarization o f this 2 Mpc sized giant. The magnetic field has a configuration predominan tly parallel to the source main axis. We observe Faraday rotation at low fr equencies, most probably produced by an ionized medium external to the radi o source, The spectral index distribution is that typical of FR II radio ga laxies, with spectral indices gradually steepening from the source extremes towards the core. Modeling the spectrum in the lobes using standard synchr otron loss models yields the spectral age of the source and the mean veloci ty of the jet-head with respect to the lobe material. The existence of a po ssible backflow in the lobe is considered to relate spectral with dynamical determinations of the age and the velocity with respect to the external me dium. Through a very simple model, we obtain a physical characterization of the jets and the external medium in which the radio galaxy expands. The re sults in 8C 0821+695 are consistent with a relativistic jet nourishing the lobes which expand in a hot, low density halo. We infer a deceleration of t he source expansion velocity which we explain through a progressive increas e in the hot-spot size.