The subarcsecond mid-infrared view of local active galactic nuclei: I.
  The N- and Q-band imaging atlas
  We present the first subarcsecond-resolution mid-infrared (MIR) atlas of
local active galactic nuclei (AGN). Our atlas contains 253 AGN with a median
redshift of z=0.016, and includes all publicly available MIR imaging performed
to date with ground-based 8-m class telescopes, a total of 895 independent
measurements. Of these, more than 60% are published here for the first time. We
detect extended nuclear emission in at least 21% of the objects, while another
19% appear clearly point-like, and the remaining objects cannot be constrained.
Where present, elongated nuclear emission aligns with the ionization cones in
Seyferts. Subarcsecond resolution allows us to isolate the AGN emission on
scales of a few tens of parsecs and to obtain nuclear photometry in multiple
filters for the objects. Median spectral energy distributions (SEDs) for the
different optical AGN types are constructed and individual MIR 12 and 18micron
continuum luminosities are computed. These range over more than six orders of
magnitude. In comparison to the arcsecond-scale MIR emission as probed by
Spitzer, the continuum emission is much lower on subarcsecond scales in many
cases. The silicate feature strength is similar on both scales and generally
appears in emission (absorption) in type I (II) AGN. However, the polycyclic
aromatic hydrocarbon emission appears weaker or absent on subarcsecond scales.
The differences of the MIR SEDs on both scales are particularly large for
AGN/starburst composites and close-by (and weak) AGN. The nucleus dominates
over the total emission of the galaxy only at luminosities >=10^(44)erg/s. The
AGN MIR atlas is well suited not only for detailed investigation of individual
sources but also for statistical studies of AGN unification.
