Unveiling the central parsec region of an active galactic nucleus: The circinus nucleus in the near-infrared with the very large telescope

VLT J- to M'-band adaptive optics observations of the Circinus galaxy on parsec scales resolve a central bright Ks-band source with a FWHM size of 1.9 ± 0.6 pc. This source is only visible at wavelengths longward of 1.6 μm and coincides in position with the peak of the [Si VII] 2.48 μm coronal line emission. With respect to the peak of the central optical emission, the source is shifted by ∼0″15 (2.8 pc) to the southeast. Indeed, the Ks-band source defines the vertex of a fairly collimated beam that extends for ∼10 pc and is seen in both continuum light shortward of 1.6 μm and in Hα line emission. The source also lies at the center of a ∼19 pc size [Si VII] ionization bicone. Identifying this source as the nucleus of Circinus, its size is compatible with a putative parsec-scale torus. Its spectral energy distribution, characterized by a prominent narrow peak, is compatible with a dust temperature of 300 K. Hotter dust within a 1 pc radius of the center is not detected. The active galactic nucleus (AGN) luminosity required to heat this dust is in the range of X-ray luminosities that have been measured toward the central source. This in turn supports the existence of highly obscuring material, with column densities of 1024 cm 2. that must be located within 1 pc of the core