The Subparsec-Scale Structure and Evolution of Centaurus A: The Nearest Active Radio Galaxy

We present high-resolution VLBI radio images of the closest classical radio galaxy, Centaurus A, including the highest resolution image yet for this source. The images were made from data obtained over a period of approximately 8 yr at the frequencies of 2.3, 4.8, and 8.4 GHz with the Southern Hemisphere VLBI Experiment (SHEVE) array and at 2.3, 8.4, and 22.2 GHz with the Very Long Baseline Array. We give complete details of the data reduction and analysis procedures. The subparsec-scale structure of Cen A is complex, consisting of a bright jet and a fainter counterjet. The bright jet contains components that have subluminal speeds of approximately 0.1c and undergo irregular episodes of rapid internal evolution. The rapid evolution sometimes observed could be interpreted as evidence for an underlying jet flow much faster (>0.45c) than observed from the proper motion of components (∼ 0.1c). Considering the large-scale morphology of the source, the motions and temporal variations in the jet, and the detection of a counterjet, we conclude that the axis of the Cen A jet lies between ∼50° and ∼80° to our line of sight. We find that the estimated times of component ejection from the compact core are reasonably coincident with enhancements in hard X-ray intensity and 22 GHz flux density. In the context of the radio galaxy population, Cen A is a low-luminosity FR I-type source and in general has the properties observed in other FR I radio galaxies. Overall, the observations of Cen A presented here, and from other investigations, are consistent with the idea that sources with an FR I appearance are not aligned with our line of sight and have relativistic flow on the subparsec scale. The apparently subluminal subparsec-scale jet components are interpreted as being slow patterns on the relativistic flow.