Mid-infrared properties of nearby low-luminosity AGN at high angular
  resolution
  High spatial resolution mid-infrared (MIR) 12 \mum continuum imaging of
low-luminosity active galactic nuclei (LLAGN) obtained by VLT/VISIR is
presented. The goal of this investigation is to determine if the nuclear MIR
emission of LLAGN is consistent with the existence of a dusty obscuring torus.
A sample of 17 nearby LLAGN was selected and combined with archival VISIR data
of 9 additional LLAGN with available X-ray measurements. Of the 17 observed
LLAGN, 7 are detected, while upper limits are derived for the 10
non-detections. All detections except NGC 3125 appear point-like on a spatial
scale of \sim 0.35". The detections do not significantly deviate from the known
MIR-X-ray correlation but extend it by a factor of \sim 10 down to luminosities
< 10^41 erg/s with a narrow scatter. The latter is dominated by the
uncertainties in the X-ray luminosity. Interestingly, a similar correlation
with comparable slope but with a normalization differing by \sim 2.6 orders of
magnitude has been found for local starburst galaxies. In addition, the VISIR
data are compared with lower spatial resolution data from Spitzer/IRS and IRAS.
By using a scaled starburst template SED and the PAH 11.3 \mum emission line
the maximum nuclear star formation contamination to the VISIR photometry is
restricted to < 30% for 75% of the LLAGN. Exceptions are NGC 1097 and NGC 1566,
which may possess unresolved strong PAH emission. Furthermore, within the
uncertainties the MIR-X-ray luminosity ratio is unchanged over more than 4
orders of magnitude in accretion rate. These results are consistent with the
existence of the dusty torus in all observed LLAGN, although the jet or
accretion disk as origin of the MIR emission cannot be excluded. Finally, the
fact that the MIR-X-ray correlation holds for all LLAGN and Seyferts makes it a
very useful empirical tool for converting between the MIR and X-ray powers of
these nuclei.
