The dusty torus in the Circinus galaxy: a dense disk and the torus funnel

Abstract

Context. With infrared interferometry it is possible to resolve the nuclear dust distributions that are commonly associated with the dusty torus in active galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus and previous interferometric observations have shown that its nuclear dust emission is particularly well resolved. Aims. The aim of the present interferometric investigation is to better constrain the dust morphology in this active nucleus. Methods. To this end, extensive new observations were carried out with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer, leading to a total of 152 correlated flux spectra and differential phases between 8 and 13 μm. To interpret this data, we used a model consisting of black-body emitters with a Gaussian brightness distribution and with dust extinction.Results. The direct analysis of the data and the modelling confirm that the emission is distributed in two distinct components: a disklike emission component with a size (FWHM) of ∼0.2 × 1.1 pc and an extended component with a size of ∼0.8 × 1.9 pc. The disk-like component is elongated along PA ∼ 46• and oriented perpendicular to the ionisation cone and outflow. The extended component is responsible for 80% of the mid-infrared emission. It is elongated along PA ∼ 107• , which is roughly perpendicular to the disk component and thus in polar direction. It is interpreted as emission from the inner funnel of an extended dust distribution and shows a strong increase in the extinction towards the south-east. We find both emission components to be consistent with dust at T ∼ 300 K, that is we find no evidence of an increase in the temperature of the dust towards the centre. From this we infer that most of the near-infrared emission probably comes from parsec scales as well. We further argue that the disk component alone is not sufficient to provide the necessary obscuration and collimation of the ionising radiation and outflow. The material responsible for this must instead be located on scales of ∼1 pc, surrounding the disk. We associate this material with the dusty torus. Conclusions. The clear separation of the dust emission into a disk-like emitter and a polar elongated source will require an adaptation of our current understanding of the dust emission in AGN. The lack of any evidence of an increase in the dust temperature towards the centre poses a challenge for the picture of a centrally heated dust distribution.

Polarized radiative transfer modeling of warped and clumpy dusty tori

“…Only the height of the structure is affected, resulting in possible polar dust signatures for intermediate and Seyfert-IIs, far beyond the torus height defined by its halfopening angle (Bock et al 2000;Burtscher et al 2013). Polar mid-infrared emission from this type of warped dust distribution could contribute to the important polar infrared flux discovered by interferometry studies of Seyfert-IIs with high position angle and baseline coverage (Tristram et al 2014;Asmus et al 2016;López-Gonzaga et al 2016). The antisymmetry between the upper and lower parts of the warped structure naturally arises even under these simple conditions.…”

“…In the midinfrared, Jaffe et al (2004) spatially resolved a parsec-sized torus-shaped distribution of dust grains in the same galaxy, where a small hot structure is embedded in a colder (320 K) dusty cocoon extending up to 3.4 pc in diameter. Further fluxlimited interferometric studies of nearby quasars, using the MIDinfrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI; see, e.g., Tristram et al 2007;Burtscher et al 2009Burtscher et al , 2013Tristram et al 2014) or the Keck interferometer (Swain et al 2003;Kishimoto et al 2009a), definitively confirmed the existence of the dusty torus, if with large differences within the sample.…”

On the Anisotropy of Nuclei Mid-Infrared Radiation in Nearby Active Galactic Nuclei

“…Infrared interferometry observations of four nearby AGNs (Circinus, NGC 1068, NGC 424, NGC 3783) at subparsec resolution suggested that, instead of the torus, the major contributor of MIR emission is dust in the polar region (Tristram et al 2007(Tristram et al , 2014Raban et al 2009;Hönig et al 2012Hönig et al , 2013. Assuming that all AGNs contain polar region dust, the dust emission in polar regions will likely dilute the anisotropic emission from the torus.…”

An X-ray variable absorber within the broad line region in Fairall 51

“…The most recent mid-infrared observations of nearby galaxies have revealed that there is a large scatter in the geometrical properties of their dusty tori (Burtscher et al 2013), and even very complicated structures of the dust within the central parsec were reported (Tristram et al 2014). …”