Observations of Cygnus a with the 5-km Radio Telescope

Maps of the distribution of radio brightness and polarization in Cygnus A at 5 GHz have been made with an angular resolution of $$2^".0\times3^".1$$ arc. These show details of the structure of the compact features in the parts of the source most distant from the optical galaxy, as well as the structure and particularly the strong polarization in parts of the extensive tails. Some limited observations at 15 GHz with a resolution of 0″.4 arc have been used to define more closely the shape of the compact components. A weak central source < 1.5 kpc in diameter has been found which lies between the two optical ‘nuclei’ in photographs of the central region of the galaxy. On the assumption of equipartition between relativistic electrons and magnetic field, and from a study of the spectral distribution, it is concluded that there must be a continuous replenishment of energetic electrons within each of the two main compact components of the source, representing an energy supply of ∼ 10⁴⁵ erg s⁻¹. These electrons must diffuse out into the extensive tail regions on a time scale ≲ 4 × 10⁴ yr and probably provide the entire energy requirements of these regions; the subsequent radiation losses provide an explanation of the change in the spectral index of the integrated emission at ∼ 1 GHz. The evidence now available is inconsistent with several of the physical models which have been proposed. The inertial and de Young & Axford models are incompatible with the upper limits on the masses of the components, as derived from the measurements of polarization, and do not account for the detailed shape of the components. Models based on a high temperature, high density ambient gas to provide the pressure needed to contain the compact components do not explain the physical alignment of the components with the central nucleus. The models most likely to provide a satisfactory explanation of the details observed seem to be those in which energy is continually transported to the components from the nucleus in the form of energetic particles or low frequency waves.