The dusty heart of nearby active galaxies I. High-spatial resolution mid-IR spectro-photometry of Seyfert galaxies

In a series of papers, we aim at stepping towards characterizing physical properties of the AGN dust torus by combining IR high-spatial resolution observations with 3D clumpy torus models. In this first paper, we present mid-IR imaging and 8-13 mu m low-resolution spectroscopy of nine type 1 and ten type 2 AGN. The observations were carried out with the VLT/VISIR mid-IR imager and spectrograph and can be considered the largest currently available mid-infrared spectro-photometric data set of AGN at spatial resolution less than or similar to 100 pc. These data resolve scales at which the emission from the dust torus dominates the overall flux, and emission from the host galaxy (e.g. star-formation) is resolved out in most cases. The silicate absorption features are moderately deep and emission features, if seen at all, are shallow. The strongest silicate emission feature in our sample shows some notable shift of the central wavelength from the expected 9.7 mu m (based on ISM extinction curves) to similar to 10.5 mu m. We compare the observed mid-IR luminosities of our objects to AGN luminosity tracers (X-ray, optical and [O III] luminosities) and find that the mid-IR radiation is emitted quite isotropically. In two cases, IC 5063 and MCG-3-34-64, we find evidence for extended dust emission in the narrow-line region. We confirm the correlation between observed silicate feature strength and Hydrogen column density, which was recently found in Spitzer data at lower spatial resolution. In a further step, our 3D clumpy torus model has been used to interpret the data. We show that the strength of the silicate feature and the mid-IR spectral index a can be used to get reasonable constraints on the radial dust distribution of the torus and the average number of clouds N-0 along an equatorial line-of-sight in clumpy torus models. The mid-IR spectral index a is almost exclusively determined by the radial dust distribution power-law index a, while the silicate feature depth mostly depends on N-0 and the torus inclination. A comparison of model predictions to our type 1 and type 2 AGN reveals that average parameters of a = -1.0 +/- 0.5 and N-0 = 5 - 8 are typically seen in the presented sample, which means that the radial dust distribution is rather shallow. As a proof-of-concept of this method, we compared the model parameters derived from a and the silicate feature strength to more detailed studies of full IR SEDs and interferometry and found that the constraints on a and N-0 are consistent. Finally, we may have found evidence that the radial structure of the torus changes from low to high AGN luminosities towards steeper dust distributions, and we discuss implications for the IR size-luminosity relation.