Abstract
Infrared (IR) interferometry has made significant progress over the last 10 years to a level that active galactic nuclei (AGN) are now routine targets for long-baseline interferometers. Almost 50 different objects have been studied today in the near-IR and mid-IR. This allowed for detailed characterisation of the dusty environment of the actively growing black holes . It was possible to show directly that the dust must be arranged in clumps, as had been indirectly inferred from theory and unresolved observations. The dust composition seems to undergo significant evolution from galactic scales to the AGN environment, with the hottest dust close to the sublimation front being dominated by large graphite grains. While the overall distribution of the dusty mass is quite diverse from object to object, indications have been found that the dust distribution may depend on AGN luminosity, with more powerful AGN potentially showing more compact dust structures. Arguably the most exciting discovery was the fact that the bulk of the mid-IR emission in Seyfert galaxies emerges from the polar region of the AGN, which is difficult to reconcile with classical torus models. An alternative model is currently being debated that consists of a dusty disc plus a dusty wind driven by radiation pressure from the central source. This finding has major implications for our understanding of AGN unification and will become a focus of the upcoming generation of instruments at the VLTI. More recently, an application of interferometry to cosmology was proposed to measure precise geometric distances to AGN in the Hubble flow. Further exploration of this method may open up interferometry to a new scientific community.
After languishing for a decade largely through lack of data, this field should now see a revival, as it is refreshed by detailed infrared imaging. The dynamical problems guessed at years ago can be brought into clearer focus. Julian Krolik, Nature News & Views, 2004
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Notes
- 1.
The label “key results” is a fully subjective judgment by the author of this chapter. They include the most cited papers in AGN IR interferometry.
- 2.
Where in all objects except of Circinus and maybe NGC 1068 we are observing in the first lobe of the Fourier transform.
- 3.
Strictly speaking, the colour temperature inferred from spectral fits is a lower limit to the physical temperature, given the uncertainty from the dust distribution, which may make the emission appear redder than the temperature in this region. However, this effect should be minor at the inner rim of the dust distribution. If we assume that the “true” temperature is 1800 K, we can expect that the maximum value the emissivity can get to is ∼ 3 for the assumption of a 1400 K black body. Similarly, the surface covering factor may be > 1 if the dust is optically thin. However, for the near-IR, this is incompatible with the red SED towards the mid-IR, for which we would expect a Rayleigh-Jeans behaviour if optically thin.
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Hönig, S.F. (2016). Tori, Discs, and Winds: The First Ten Years of AGN Interferometry. In: Boffin, H., Hussain, G., Berger, JP., Schmidtobreick, L. (eds) Astronomy at High Angular Resolution. Astrophysics and Space Science Library, vol 439. Springer, Cham. https://doi.org/10.1007/978-3-319-39739-9_6
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