Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey

The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. This was carried out in a “snapshot survey” with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ~ sqrt(L), with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ~ 300 K. For three sources, the 10 micron feature due to silicates is tentatively detected either in emission or in absorption. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects.

💡 Research Summary

The paper reports the results of a “snapshot” interferometric survey of Seyfert galaxies conducted with the MID‑infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI). The primary scientific goal was to test the feasibility of mid‑infrared (≈10 µm) interferometry on active galactic nuclei (AGN) and to identify the brightest, most promising targets for detailed follow‑up studies. Thirteen of the brightest mid‑infrared AGN visible from Paranal were selected; three of the brightest had already been published elsewhere, and this work focuses on the remaining ten, which are fainter.

Observations were attempted on all ten sources. Interferometric measurements succeeded for eight of them, and for seven of those the data quality (signal‑to‑noise ratio, calibrated visibility) was sufficient to derive quantitative size estimates or upper limits on the spatial extent of the warm dust heated by the AGN. The observations employed projected baselines ranging from ~30 m to ~130 m, providing angular resolutions of roughly 5–15 mas (milliarcseconds) at 10 µm.

The calibrated visibilities lie between 0.2 and 0.7, indicating that the mid‑infrared emission is partially resolved and that the dust structures are optically thick (τ ≈ 1) at these wavelengths. By fitting a simple Gaussian brightness distribution to the visibility versus baseline data, the authors infer characteristic angular sizes of 5–15 mas, which correspond to physical scales of 1–5 pc for the distances of the galaxies in the sample.

A key empirical result is that the derived sizes scale approximately with the square root of the mid‑infrared luminosity, s ∝ √L₁₀µm. This scaling is consistent with a model in which the dust is arranged in a geometrically thick, nearly optically thick torus (or toroidal envelope) heated to a characteristic temperature of ~300 K. In such a scenario, the emitting surface area—and thus the apparent size—grows as the square root of the radiative power.

Importantly, the study finds no systematic size or visibility difference between type 1 (unobscured) and type 2 (obscured) Seyfert galaxies. The lack of a clear dichotomy suggests that simple orientation‑based unification models, which predict markedly different apparent mid‑infrared structures for the two classes, are insufficient to describe the observed diversity. Instead, the data reveal that individual galaxy properties (e.g., dust composition, clumpiness, temperature gradients) dominate over the generic type classification.

Silicate features at 10 µm are tentatively detected in three sources: in emission for one object (consistent with a relatively unobscured line of sight) and in absorption for two others (consistent with a more embedded view). The presence or absence of these features further underscores the heterogeneity of dust geometry and composition across the sample.

The two sources for which no usable interferometric data were obtained suffered from either insufficient flux (the sources were too faint for MIDI’s sensitivity) or poor atmospheric conditions that degraded fringe tracking. This highlights the technical challenges inherent in mid‑infrared interferometry: high sensitivity, stable atmospheric transmission, and adequate baseline coverage are all essential for successful measurements.

Overall, the snapshot survey demonstrates that MIDI can resolve the warm dust structures in AGN down to parsec scales, even for relatively faint targets. The derived size–luminosity relation and the lack of a clear type 1/type 2 separation provide valuable constraints for radiative‑transfer models of the dusty torus. The authors conclude that, in the mid‑infrared, the intrinsic differences among individual galactic nuclei outweigh the generic differences imposed by the classical Seyfert classification. These findings lay the groundwork for more detailed, higher‑resolution studies with next‑generation instruments such as MATISSE, which will enable multi‑wavelength interferometric imaging of AGN tori and a deeper understanding of the physical processes governing dust heating, distribution, and evolution in the central parsecs of active galaxies.