2017ApJ...835..169O

We performed near-diffraction-limited (≃0′′ / . 4 FWHM) N-band imaging of one of the nearest active galactic nuclei (AGNs) in M51 with the 8.2 m Subaru Telescope to study the nuclear structure and spectral energy distribution (SED) at 8-13 µm. We found that the nucleus is composed of an unresolved core (at ≃13 pc resolution, orintrinsic size corrected for the instrumental effect of 6 pc) and an extended halo (at a few tens of parsec 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 deficient polycyclic aromatic hydrocarbon emission due to the AGN. The core SED is likely dominated by the AGN because of the following two reasons. First, the clumpy torus model SEDs can reproduce the red mid-infrared continuum with apparently moderate silicate 9.7 µm absorption. Second, the core 12 µm 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.