Mid-infrared interferometry of nearby Active Galactic Nuclei

Abstract

Active galactic nuclei are powered by the accretion of matter onto supermassive black holes in the centre of galaxies. In unified schemes, warm, obscuring dust and gas in a torus play a major role for the different manifestations of such nuclei. The differences are thought to depend on the viewing angles with respect to the dust distribution, which absorbs the nuclear light and re-emits it in the infrared. The scientific goal of this thesis is to reveal the nature of the nuclear mid-infrared source in a few nearby active galaxies in order to explore the innermost dust distribution on parsec scales. As this requires angular resolutions of < 20 mas, interferometric observations with the MIDI instrument at the Very Large Telescope Interferometer were performed. The primary target is the Circinus galaxy, a prototype Seyfert 2 galaxy. The modelling of the interferometric data revealed that warm dust (T ~ 300 K) is located in two components: in a disk with a radius of 0.2 pc and in a larger, geometrically thick and clumpy torus extending out to 1 pc. The size, orientation and temperature of this configuration are in agreement with the long sought dusty torus of Seyfert galaxies. The mid-infrared emission in the radio galaxy Centaurus A is of a totally different nature. The bulk of its emission comes from an unresolved synchrotron core, most likely the foot of the jet. Only about 30% of the mid-infrared flux originates in an extended distribution of warm dust. Three further targets observed by MIDI (Mrk 1239, MCG-05-23-016 and 3C 273) are basically unresolved, only allowing upper limits on the size of their dust distributions. Although the completely new observations in general confirm the concept of unification, significant discrepancies between the observations and very simple models occur which need to be quantified by further (interferometric) observations. This may lead to a revision of the most simple unified schemes.