  We performed near-diffraction-limited (~0.4 FWHM) N-band imaging of one of
the nearest Active Galactic Nucleus (AGN) in M51 with 8.2m Subaru telescope to
study the nuclear structure and spectral energy distribution (SED) at 8-13 um.
We found that the nucleus is composed of an unresolved core (at ~13 pc
resolution, or intrinsic size corrected for the instrumental effect of <6 pc)
and an extended halo (at a few tens pc scale), and each of their SEDs is almost
flat. We examined the SED by comparing with the archival Spitzer IRS spectrum
processed to mimic our chopping observation of the nucleus, and the published
radiative-transfer model SEDs of the AGN clumpy dusty torus. The halo SED is
likely due to circumnuclear star formation showing little Polycyclic Aromatic
Hydrocarbon (PAH) emission due to the AGN. The core SED is likely dominated by
the AGN because of the following two reasons. Firstly, the clumpy torus model
SEDs can reproduce the red mid-infrared continuum with apparently moderate
silicate 9.7 um absorption. Secondly, the core 12 um luminosity and the
absorption-corrected X-ray luminosity at 2-10 keV in the literature follow the
mid-infrared-X-ray luminosity correlation known for the nearby AGNs including
the Compton-thick ones.
