The dusty heart of nearby active galaxies. I. High-spatial resolution
  mid-IR spectro-photometry of Seyfert galaxies
  We present 8-13 micron imaging and spectroscopy of 9 type 1 and 10 type 2 AGN
obtained with the VLT/VISIR instrument at spatial resolution <100 pc. The
emission from the host galaxy sources is resolved out in most cases. The
silicate absorption features are moderately deep and emission features are
shallow. We compare the mid-IR luminosities to AGN luminosity tracers and found
that the mid-IR radiation is emitted quite isotropically. In two cases, IC5063
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 recently found in Spitzer data. 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 can be used to
get reasonable constraints on the dust distribution in the torus. The mid-IR
spectral index, alpha, is almost exclusively determined by the radial dust
distribution power-law index, a, and the silicate feature depth is mostly
depending on the average number of clouds, N0, along an equatorial
line-of-sight and the torus inclination. A comparison of model predictions to
our type 1 and type 2 AGN reveals typical average parameters a=-1.0+/-0.5 and
N0=5-8, 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 to more detailed studies of IR SEDs and
interferometry and found that the constraints on a and N0 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. (abridged)
