Formation of cavities in the X-ray emitting cluster gas of Cygnus A

Abstract

The ROSAT data from the cluster gas of Cygnus A are re-examined in light of a three-dimensional hydrodynamic simulation of a supersonic jet propagating in an isothermal King atmosphere designed to follow the observed density distribution of the Cygnus A cluster gas. In addition to the presence of a non-uniform atmosphere, this simulation departs from previous 3D work in that the X-ray (bremsstrahlung) brightness distribution has been determined from the flow variables as a function of time. This simulation provides compelling evidence for the conclusions reached by Carilli et al. who stated that the major features of the X-ray brightness distribution are manifestations of the expanding lobe and the passage of a bow shock within the atmosphere. Further, this work demonstrates that these X-ray features are as much a function of the observing frequency and bandwidth as they are of the local conditions in the cluster gas of Cygnus A. In addition, new estimates of the Mach number of the jet (M≳4) as well as the density and temperature jumps across the bow shock are derived by comparing the simulations with the X-ray data from Cygnus A.