The subparsec-scale structure and evolution of Centaurus A II : continued very long baseline array monitoring

We present the results of continued 8.4 GHz Very Long Baseline Array monitoring of the subparsec-scale structure and evolution of Centaurus A, following on from the initial results presented in 1998 by Tingay et al. We include, for the first time, multiepoch VLBI images at 22.2 GHz that show that the jet is linear and well collimated on scales as small as 0.02 pc (~1000rs). Two components in the subparsec-scale jet continue to evolve slowly with a speed of 0.12c. We confirm that an additional component, close to the core, has no significant motion. Some evidence is seen for rapid variations within individual components, as noted dramatically in 1991 - 1992 by Tingay et al., albeit at a lower level of activity. Both the stationary behavior of the component close to the core and the internal variability of components in the subparsec-scale jet of Centaurus A may be explained as being due to the existence of shocks created in the wake of major component ejections from the nucleus, as simulated by Agudo et al. (published in 2001). Tentative evidence is found to suggest that two subparsec-scale counterjet components are in motion away from the nucleus. The estimated apparent speeds of the jet and counterjet components are consistent with the previously suggested likely jet viewing angle range, 50° - 80°. We also compare our Centaurus A images with high-resolution VLBI images of M87 to show that the region of the Centaurus A jet in which collimation likely first occurs lies a factor of 10 below our current resolution limit. Future space VLBI missions at high frequency will be required to resolve this region