Sub-parsec scale imaging of Centaurus A (1006.1486v3)

Abstract: At a distance of about 3.8 Mpc, the radio galaxy Centaurus A is the closest active galaxy. Therefore it is a key target for studying the innermost regions of active galactic nuclei (AGN). VLBI observations conducted within the framework of the TANAMI program enable us to study the central region of the Cen A jet with some of the highest linear resolutions ever achieved in an AGN. This region is the likely origin of the gamma-ray emission recently detected by the Fermi Large Area Telescope (LAT). TANAMI monitors a sample of radio and gamma-ray selected extragalactic jets south of -30 degrees declination at 8.4 GHz and 22.3 GHz with the Australian Long Baseline Array (LBA) and the transoceanic antennas Hartebeesthoek in South Africa, the 6 m Transportable Integrated Geodetic Observatory (TIGO) in Chile and the 9 m German Antarctic Receiving Station (GARS) in O'Higgins, Antarctica. The highest angular resolution achieved at 8.4 GHz in the case of Cen A is 0.59mas x 0.978mas (natural weighting) corresponding to a linear scale of less than 18 milliparsec. We show images of the first three TANAMI 8.4 GHz observation epochs of the sub-parsec scale jet-counterjet system of Cen A. With a simultaneous 22.3 GHz observation in 2008 November, we present a high resolution spectral index map of the inner few milliarcseconds of the jet probing the putative emission region of gamma-ray-photons.