Ground-Based Mid-Infrared Study of the Compton-Thick AGN in M51 at 10-100 pc Scale

TL;DR

This study uses high-resolution mid-infrared imaging to analyze the nuclear structure of the Compton-thick AGN in M51, revealing a compact core and extended halo with distinct spectral properties, supporting the presence of an active nucleus and circumnuclear star formation.

Contribution

First high-resolution mid-infrared imaging of M51's AGN at 8-13 um, resolving core and halo structures and analyzing their spectral energy distributions in detail.

Findings

The nucleus has an unresolved core (~6 pc size) and an extended halo.

The core's SED is dominated by the AGN, consistent with clumpy torus models.

The halo's SED likely originates from circumnuclear star formation.

Abstract

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.