The dusty heart of nearby active galaxies I . From 3 D clumpy torus models to physical properties of dust around AGN

With the possibilities of high-spatial resolution imaging and spectroscopy as well as infrared (IR) interferometry, the dusty environments of active galactic nuclei (AGN) are now in reach of observations. The presumed optically-and geometrically-thick dusty structure, or " dust torus " , is located around the mid-plane of the nuclear accretion disk, reaching from sub-parsec scales outward to several to tens of parsecs. In a series of papers, we aim for stepping towards characterizing physical properties of the dust torus by combining IR high-resolution observations with 3D radiative transfer models. Our main goals involve constraining the spatial dis-tribuion of the dust density, exploring the chemical composition of the dust around the AGN, and studying the structure of the dust cloud distribution. In this first paper, we introduce an upgrade to our 3-dimensional radiative transfer model of clumpy dust tori. The models are presented as tools to translate classical and interferometric observations into characteristical parameters of the dust distribution. We compare model SEDs for different chemical and grain-size compositions of the dust and find that clouds with standard ISM dust and optical depth τ V ∼ 50 appear in overall agreement with observed IR SEDs. By studying parameter dependencies, it is shown that type 1 AGN SEDs can be used to constrain the radial dust cloud distribution power-law index a, as well as the mean number of clouds along an equatorial line of sight N 0 which characterizes the obscuration. Possible degeneracies with the vertical structure of the torus can be overcome by using interferometric data – either modeling the baseline-dependence or the wavelength-dependence of the visibility. Although type 2 AGN can, in principle, also be used to constrain model parameters, obscuration effects make the analysis more ambiguous. We propose a simple, interferometry-based method to distinguish between " compact " and " extended " radial dust distributions without detailed modeling of the data.