The dusty heart of Circinus

I. Imaging the circumnuclear dust in N-band^⋆,⋆⋆

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
e-mail: isbell@mpia-hd.mpg.de
² Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, Gent 9000, Belgium
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de México 04510, Mexico
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁸ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1090 GE Amsterdam, The Netherlands
¹⁰ European Southern Observatory Headquarters, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany

Received: 4 February 2022
Accepted: 26 April 2022

Abstract

Context. Active galactic nuclei play a key role in the evolution of galaxies, but their inner workings and physical connection to the host are poorly understood due to a lack of angular resolution. Infrared interferometry makes it possible to resolve the circumnuclear dust in the nearby Seyfert 2 galaxy, the Circinus Galaxy. Previous observations have revealed complex structures and polar dust emission but interpretation was limited to simple models. The new Multi AperTure mid-Infrared Spectro-Scopic Experiment (MATISSE) makes it possible to image these structures for the first time.

Aims. We aim to precisely map the morphology and temperature of the dust surrounding the supermassive black hole through interferometric imaging.

Methods. We observed the Circinus Galaxy with MATISSE at the Very Large Telescope Interferometer (VLTI), producing 150 correlated flux spectra and 100 closure phase spectra. The novel inclusion of closure phases makes interferometric imaging possible for the first time. We reconstructed images in the N-band at ∼10 mas resolution. We fit blackbody functions with dust extinction to several aperture-extracted fluxes from the images to produce a temperature distribution of central dusty structures.

Results. We find significant substructure in the circumnuclear dust: central unresolved flux of ∼0.5 Jy, a thin disk 1.9 pc in diameter oriented along ∼45°, and a ∼4 × 1.5 pc polar emission extending orthogonal to the disk. The polar emission exhibits patchiness, which we attribute to clumpy dust. Flux enhancements to the east and west of the disk are seen for the first time. We distinguish the temperature profiles of the disk and of the polar emission: the disk shows a steep temperature gradient indicative of denser material; the polar profile is flatter, indicating clumpiness and/or lower dust density. The unresolved flux is fitted with a high temperature, ∼370 K. The polar dust remains warm (∼200 K) out to 1.5 pc from the disk. We attribute approximately 60% of the 12 μm flux to the polar dust, 10% to the disk, and 6% is unresolved; the remaining flux was resolved out. The recovered morphology and temperature distribution resembles modeling of accretion disks with radiation-driven winds at large scales, but we placed new constraints on the subparsec dust.

Conclusions. The spatially resolved subparsec features imaged here place new constraints on the physical modeling of circumnuclear dust in active galaxies; we show strong evidence that the polar emission consists of dust clumps or filaments. The dynamics of the structures and their role in the Unified Model remain to be explored.