Mid-infrared interferometry of Active Galactic Nuclei: an outstanding scientific success of the VLTI

Active Galactic Nuclei (AGN) are powered by accretion onto a supermassive black hole. The unified scheme for strongly accreting AGN postulates that the central engine is enshrouded by a doughnut-shaped structure of gas and dust the so-called torus. We report observations with the MID-Infrared Interferometric Instrument (MIDI) at the VLT Interferometer, which resolve the tori in the nearest Seyfert 2 galaxies, and suggest a complex structure, consisting of a compact inner disc embedded in a patchy or filamentary outer torus. The prominent nearby radio galaxy Centaurus A, however, shows little sign of a torus. Instead, its mid-infrared emission is dominated by non-thermal radiation from the base of the radio jet. Thus, not all classes of AGN contain a thick torus