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Abstract

Active galactic nuclei (AGN) play a crucial role in the formation and evolution of their host galaxies. Understanding the dust in the vicinity of supermassive black holes is key to understanding how AGN are fed and how they interact with their hosts. This circumnuclear dust traces both radiation-driven outflows and dense molecular gas which feeds the AGN.Circumnuclear dust structures are also thought to be responsible for determining AGN types and are a key feature of AGN unification. In this thesis, I use mid-infrared observations of circumnuclear dust to reveal its morphology and temperature. I use the new mid-infrared interferometer, MATISSE, to observe NGC 1068 and the Circinus Galaxy, two of the nearest AGN. Using these observations, I have produced the very first mid-infrared images of the circumnuclear dust. Analysis of these images in the L-, M-, and N-bands reveals the temperature and structure of the circumnuclear dust at sub-parsec resolution. In both galaxies, the images show a disk-like substructure which obscures emission from the accretion disk. Perpendicular to the disk -- in the polar direction -- warm dust extends for several parsecs. The temperature of this polar dust remains high (> 200 K) at large distances from the accretion disk, indicating significant clumpiness of the dust. Additionally, substructures revealed in the images provide direct evidence for clumpy or filamentary polar dust. To complement the high-resolution study of these two AGN, I compiled a low-resolution catalog of 119 AGN. The reported L- and M-band fluxes indicate 44 targets for further high-resolution study with MATISSE. The L-M and M-N colors are compared to various radiative transfer models of the circumnuclear dust and show that models with clumpy polar dust provide the best match. Together the high- and low-resolution studies indicate a disk+wind model of the circumnuclear dust: the disk is a geometrically-thin obscuring structure, which plays a role in the Seyfert 1/2 dichotomy and is associated with the inflowing material that feeds the AGN; the wind (shown via the polar dust) is clumpy, warm, and associated with massive outflows from the AGN to the host galaxy. The combination of low-resolution mid-infrared fluxes/colors with the first sub-parsec images of the circumnuclear dust places strong constraints on both future modeling and AGN unification.