The Cygnus A Jet: Parabolic Streamlines up to Kiloparsec Scales

We investigated the distance dependence of the transverse width of the jet W(r) of Cygnus A, one of the typical Fanaroff-Riley type II radio galaxies; the radial (deprojected) distance r ranged from 0.28 pc to 60 kpc (from 1.1 × 10³ to 2.4 × 10⁸ R _S) on both the approaching and counter-jet sides. The power-law dependences (W(r) ∝ r ^a ) of the jet-width profile in the parsec-scale and kiloparsec-scale regions consistently show a ∼ 0.55. For the first time, we have discovered that active collimation, i.e., a smooth reduction in the opening angle with increasing distance, persists up to distances beyond the Bondi radius. We have also discovered an apparent discontinuity in the radial profiles of the jet width, by which the jet downstream appears to widen a factor of six compared with the expectation from extrapolating the upstream profile, at the radial distance between 57 and 170 pc (from 2.3 × 10⁵ to 6.8 × 10⁵ R _S, including the Bondi radius for Cygnus A), which is a blank in our measurements. We discuss possible explanations. The cross-section of jets was physically inflated and collimation again owing to the loss of dynamic equilibrium or a recollimation shock in a relatively flat profile of ambient pressure. We also discuss an alternative possibility to explain the observed discontinuity by a multi-layered structure in the jets: an outer (slower) layer and an inner (faster) layer with flows that are progressively accelerated with distance, throughout the parabolic streamline. The outer layer would be in lower brightness at small distances, whereas the inner layer attenuates at large distances, due to Doppler debeaming as a highly inclined jet of Cygnus A.