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 K s -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 K s -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 10 24 cm -2 , that must be located within 1 pc of the core.