Resolving the jet in Cygnus A

Abstract

Cygnus A is the closest (z = 0.057) strong FR II radio galaxy and therefore a key object for detailed studies of its prominent double sided jet and nucleus. Owing to the large inclination of the jet with respect to the observer (> 75◦), and correspondingly reduced relativistic effects which allow to measure directly the jet speed, Cyg A is an ideal candidate for detailed studies of its jet physics, which is thought to be similar to those in the more luminous quasars. Our previous studies revealed a good kinematic model for the jet of Cygnus A, but the counter-jet speed is still not well constrained. The central engine and part of the counter-jet of Cyg A are likely to be obscured by free-free absorbing material, presumably a thick torus. At mm-wavelengths, the absorber becomes optically thin, which provides a more detailed view into the inner nuclear region. Knowing the speed of jet and counter-jet and their flux density ratio allows to determine the jet Lorentz factors and orientation. Therefore we started to monitor Cyg A with global VLBI at 43 GHz in Oct. 2007. Our first epoch reveals a previously unseen gap between both jets. This could be either a sign for a new counter-jet component that is slowly separating or we start to see the very inner acceleration region of the jet which is not efficiently radiating at radio wavelengths. Further more the image shows transversely resolved jet structures at distances beyond ∼ 0.5 pc which facilitate more detailed investigations addressing jet stratification. Analysis of the resolved jet structure shows that the initially wide jet (opening angle ∼ 10◦) collimates within the first parsec into a edge-brightened jet with an opening angle of ∼ 3◦.