Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 472, Issue 4, Pages 3854-3870Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx2227
Keywords
radiative transfer; galaxies: active; galaxies: individual: Circinus; galaxies: nuclei; galaxies: Seyfert
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Funding
- Ministry of Education, Science and Technological Development of the Republic of Serbia through the project Astrophysical Spectroscopy of Extragalactic Objects [176001]
- FONDECYT [3140518]
- Center of Excellence in Astrophysics and Associated Technologies
- Ministry of Education, Science and Technological Development of the Republic of Serbia through the project Large Scale Structure of the Universe [176003]
- European Southern Observatory (ESO) [60.A-9629, 076.B-0599, 078.B-0255, 087.B-0746, 088.B-0159, 089.B-0070, 385.B-0896, 386.B-0026]
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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 disc and a large-scale dusty cone shell, illuminated by a tilted accretion disc 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 the AGN.
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