2012ApJ...755..149H

Advancements in infrared (IR) interferometry open up the possibility to spatially resolve active galactic nuclei (AGNs) on the parsec-scale level and study the circumnuclear dust distribution, commonly referred to as the "dust torus," that is held responsible for the type 1/type 2 dichotomy of AGNs. We used the mid-IR beam combiner MIDI together with the 8 m telescopes at the Very Large Telescope Interferometer to observe the nucleus of the Seyfert 2 galaxy NGC 424, achieving an almost complete coverage of the uv-plane accessible by the available telescope configurations. We detect extended mid-IR emission with a relatively baseline- and model-independent mid-IR half-light radius of (2.0±0.2) pcx(1.5±0.3) pc (averaged over the 8-13 µm wavelength range). The extended mid-IR source shows an increasing size with wavelength. These properties are in agreement with the idea of dust heated in thermal equilibrium with the AGN. The orientation of the major axis in position angle ∼ - 27° is closely aligned with the system axis as set by optical polarization observations. Torus models typically favor extension along the mid-plane at mid-IR wavelengths instead. Therefore, we conclude that the majority of the parsec-scale mid-IR emission ( ≳ 60%) in this type 2 AGN originates from optically thin dust in the polar region of the AGN, a scenario consistent with the near- to far-IR spectral energy distribution. We suggest that a radiatively driven dusty wind, possibly launched in a puffed-up region of the inner hot part of the torus, is responsible for the polar dust. In this picture, the torus dominates the near-IR emission up to about 5 µm, while the polar dust is the main contributor to the mid-IR flux. Our results of NGC 424 are consistent with recent observations of the AGN in the Circinus galaxy and resemble large-scale characteristics of other objects. If our results reflect a general property of the AGN population, the current paradigm for interpreting and modeling the IR emission of AGNs has to be revised.