[Paper Review] The dusty heart of Circinus: I. Imaging the circumnuclear dust in N-band

J. W. Isbell, C. Paladini+29 more|Ghent University Academic Bibliography (Ghent University)|May 3, 2022

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses VLTI/MATISSE N-band interferometric imaging to resolve the circumnuclear dust in the Circinus Galaxy for the first time, revealing a thin 1.9 pc disk at 45° orientation and a patchy, 4×1.5 pc polar emission structure. The polar dust contributes ~60% of the 12 μm flux and exhibits a flatter temperature profile than the denser disk, providing direct evidence for clumpy, radiation-driven outflow structures in AGN unification models.

ABSTRACT

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, Circinus. Previous observations have revealed complex structures and polar dust emission but interpretation was limited to simple models. MATISSE makes it possible to image these structures for the first time. We observed the Circinus Galaxy with VLTI/MATISSE, producing 150 correlated flux spectra and 100 closure phase spectra. 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. 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 deg,and a ~4x1.5 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. 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. The subparsec features imaged here place new constraints on the physical modeling of circumnuclear dust in active galaxies.

Motivation & Objective

• To map the morphology and temperature distribution of circumnuclear dust in the Circinus Galaxy at subparsec resolution.
• To test the validity of unified AGN models by resolving dust structures around the supermassive black hole.
• To determine the contribution of different dust components—central unresolved flux, disk, and polar emission—to the total mid-infrared flux.
• To constrain physical models of the dusty torus and radiation-driven winds using spatially resolved interferometric data.
• To investigate the clumpiness and inhomogeneity of polar dust emission through closure phase imaging.

Proposed method

• Observations were conducted with the Multi AperTure mid-Infrared Spectro-Imaging Experiment (MATISSE) at the Very Large Telescope Interferometer (VLTI).
• 150 correlated flux spectra and 100 closure phase spectra were recorded, enabling interferometric imaging at ~10 mas resolution.
• Image reconstruction was performed using visibility and closure phase data to recover the spatial morphology of the N-band dust emission.
• Blackbody functions with dust extinction were fitted to aperture-extracted fluxes from the reconstructed images to derive temperature and extinction profiles.
• The temperature distribution was analyzed across 13 apertures to distinguish radial gradients in the disk and polar extension.
• Model comparisons were made with clumpy torus models (Schartmann et al. 2008; Stalevski et al. 2019) and disk+wind simulations (Wada et al. 2016).

Experimental results

Research questions

• RQ1 What is the spatial morphology of the circumnuclear dust in the Circinus Galaxy at subparsec resolution?
• RQ2 How do the temperature profiles of the disk and polar dust components differ, and what do they reveal about dust density and clumpiness?
• RQ3 To what extent does the polar dust emission consist of clumps or filaments, and how does this affect AGN unification models?
• RQ4 What fraction of the 12 μm flux originates from the central unresolved source, the disk, and the polar structure?
• RQ5 How well do existing clumpy torus and disk+wind models reproduce the observed morphology and temperature distribution?

Key findings

• The circumnuclear dust structure consists of a central unresolved flux of ~0.5 Jy, a thin disk 1.9 pc in diameter oriented at ~45°, and a polar emission extending ~4×1.5 pc orthogonal to the disk.
• The polar dust emission exhibits significant patchiness on the scale of the resolution element, providing direct evidence for clumpiness rather than a smooth, continuous structure.
• The polar dust contributes approximately 60% of the total 12 μm flux, with the disk contributing 10% and the unresolved core 6%, while the remaining flux was resolved out.
• The central unresolved flux is fitted with a high temperature of ~370 K, while the polar dust remains warm (T ~ 200 K) up to 1.5 pc from the center, indicating sustained heating.
• The disk shows a steep temperature gradient, indicating dense, compact material, whereas the polar component exhibits a flatter temperature profile, consistent with clumpiness and/or lower density.
• The dust extinction is well-fit by a foreground screen with A_V = 28.5_{-7.7}^{+8.5} mag, consistent with galactic-scale values and showing no significant local variation across the field.