Dissecting the active galactic nucleus in Circinus -- I. Peculiar mid-IR
  morphology explained by a dusty hollow cone
  Recent high angular resolution observations resolved for the first time the
mid-infrared (MIR) structure of nearby active galactic nuclei (AGN).
Surprisingly, they revealed that a major fraction of their MIR emission comes
from the polar regions. This is at odds with the expectation based on AGN
unification, which postulates a dusty torus in the equatorial region. The
nearby, archetypical AGN in the Circinus galaxy offers one of the best
opportunities to study the MIR emission in greater detail. New, high quality
MIR images obtained with the upgraded VISIR instrument at the Very Large
Telescope show that the previously detected bar-like structure extends up to at
least 40 pc on both sides of the nucleus along the edges of the ionization
cone. Motivated by observations across a wide wavelength range and on different
spatial scales, we propose a phenomenological dust emission model for the AGN
in the Circinus galaxy consisting of a compact dusty disk and a large-scale
dusty cone shell, illuminated by a tilted accretion disk with an anisotropic
emission pattern. Undertaking detailed radiative transfer simulations, we
demonstrate that such a model is able to explain the peculiar MIR morphology
and account for the entire IR spectral energy distribution. Our results call
for caution when attributing dust emission of unresolved sources entirely to
the torus and warrant further investigation of the MIR emission in the polar
regions of AGN.
