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 9 type 1 and 10 type 2 AGN .
The observations were carried out using 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 \la 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 \sim 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 found that the mid-IR radiation is emitted quite isotropically .
In two cases , ICÂ 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 \alpha 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 \alpha is almost exclusively determined by the radial dust distribution power-law index a and 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 \pm 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 \alpha 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 might 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 .