2006PASJ...58..211H

We present the results of a VLBI Space Observatory Programme (VSOP) observation of the subparsec structure in Cen A at 4.9GHz. The observation produced an image of the subparsec jet components with a resolution of three-times better than images from previous VLBI monitoring campaigns at 8.4GHz, and twice better than the previous 22GHz studies. Owing to its proximity, our Cen A space-VLBI image is one of the highest spatial-resolution images of an AGN ever made – 0.01pc per beam – comparable only to the recent 43GHz VLBI images of M87. The elongated core region is resolved into several components of over 10 milliarcsec long (0.2pc), including a compact component of brightness temperature 2.2 x 10¹⁰ K. A counterjet was detected: if we assume jet-counterjet symmetry, a relatively slow jet speed, and a large viewing angle, as derived from previous observations, the image allows us to investigate the distribution of ionized gas around the core, which is opaque at this frequency due to free-free absorption. We also analyzed the jet geometry in terms of collimation. Assuming the strongest component to be the core, the jet opening angle at ∼ 5000 r_S from the core is estimated to be ∼ 12°, with the collimation of the jet to ∼ 3° continuing out to ∼ 20000 r_S. This result is consistent with previous studies of the jet in M87, which favor MHD disk outflow models. Future space VLBI observations at higher frequencies will probably be able to image the collimation region, within 1000 r_S of the center of Cen A, together with the accretion disk itself.