Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy

Abstract

Aims. To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared. Methods. Observations were obtained with the MIDI instrument at the Very Large Telescope Interferometer. The 21 visibility points recorded are dispersed with a spectral resolution of λ/δλ ≈ 30 in the wavelength range from 8 to 13 µm. To interpret the data we used a stepwise approach of modelling with increasing complexity. The final model consists of two black body Gaussian distributions with dust extinction. Results. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 µm in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk of L acc = 10 10 L , which corresponds to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Conclusions. Based on a great leap forward in the quality and quantity of interferometric data, our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or "clumpy".

Modeling optical and UV polarization of AGNs

“…The interferometric observations of the type-2 Circinus galaxy undertaken by Tristram et al (2007) with the midinfrared (MIDI) instrument at the Very Large Telescope strongly support the presence of a clumpy toroidal structure. The data are inconsistent with a smooth density distribution and indicate clumpy or filamentary structures.…”

Synapses of active galactic nuclei:

“…The torus must not be spherically symmetric to obscure the BLR, so that at the same time the region producing the permitted narrow lines (known as narrow-line region, NLR) reaches us from the same LOS. The locus of this obscuring material was initially postulated at parsec scales and confirmed by modelling the spectral energy distribution (SED) of Seyferts (e.g., Ramos Almeida et al 2011;Alonso-Herrero et al 2011) and by interferometric observations (e.g., Circinus galaxy, Tristram et al 2007). Such scales are beyond the current instrumentation, therefore the torus morphology can only be inferred by indirect measurements.…”

The Co-Evolution of Galaxies and Black Holes: Current Status and Future Prospects

“…1. With JWST and an AO-fed GSMT/E-ELT, we will just be able access this regime and map the emission of hot dust (Tristram et al 2007) and hot molecular hydrogen (Davies et al 2006;Hicks & Malkan 2008). On larger spatial scales, JWST, GSMT/E-ELT, and ALMA could be used to map out the distribution and kinematics of ionized and molecular gas for a large and complete sample of AGN and suitable control sample (e.g., Fathi et al 2006;Storchi-Bergmann et al 2007;Riffel et al 2008;Davies et al 2009a;Lindt-Krieg et al 2008;Schinnerer et al 2000).…”