JWST reveals the diversity of nuclear obscuring dust in nearby AGN: nuclear isolation of MIRI/MRS datacubes and continuum spectral fitting

Omaira González-Martín, Daniel J. Díaz-González, Mariela Martínez-Paredes, Almudena Alonso-Herrero, Enrique López-Rodríguez, Begoña García-Lorenzo, Cristina Ramos Almeida, Ismael García-Bernete, Donaji Esparza-Arredondo, Sebastian F. Hoenig, Santiago García-Burillo, Chris Packham, Nancy A. Levenson, Alvaro Labiano, Miguel Pereira-Santaella, Francoise Combes, Anelise Audibert, Erin K. S. Hicks, Lulu Zhang, Enrica Bellocchi, Richard I. Davies, Laura Hermosa Muñoz, Masatoshi Imanishi, Claudio Ricci, Marko Stalevski

First listed 2025-04-01 | Last updated 2026-04-13

Abstract

We investigate the capabilities of the mid-infrared instrument (MIRI) of James Webb Space Telescope (JWST) to advance our knowledge of AGN dust using the spectral fitting technique on an AGN collection of 21 nearby (z<0.05) AGN (7 type-1 and 14 type-2) observations obtained with the medium resolution spectroscopy (MRS) mode. This collection includes publicly available AGN and data from the collaboration of Galactic Activity, Torus, and Outflow Survey (GATOS). We developed a tool named MRSPSFisol that decomposes MRS cubes into point-like and extended contributions. We found statistically good fits for 12 targets with current AGN dust models. The model that provides good fits (chi2/dof<2) for {these 12 targets} assumes a combination of clumpy and smooth distribution of dust in a flare-disk geometry where the dust grain size is a free parameter. Still, two and one AGN statistically prefer the disk+wind and the classical clumpy torus model, respectively. However, the currently available models fail to reproduce 40% of the targets, likely due to the extreme silicate features not well reproduced by the models and signatures of water-ice and aliphatic hydrocarbon absorption features in most targets. New models exploring, for instance, new chemistry, are needed to explain the complexity of AGN dust continuum emission observed by JWST.

Short digest

The authors PSF-isolate nuclear mid-IR emission in 21 nearby (z<0.05) AGN with a new tool (MRSPSFisol) and fit the continuum using multiple dust libraries. Twelve nuclei achieve good fits (chi2/dof<2), typically with a flared-disk model combining clumpy and smooth dust and allowing grain size to vary; two prefer disk+wind and one the classical clumpy torus. About 40% resist all models because of extreme silicate profiles plus pervasive water-ice and aliphatic hydrocarbon absorption, pointing to missing chemistry. This establishes JWST/MRS as a clean nuclear isolator and shows that next-generation dust models must capture more complex compositions.

Key figures to inspect

• MRSPSFisol demonstration: nuclear vs extended spectra/maps across the four MRS channels, showing removal of PAH-dominated host emission and the recovered nuclear flux fraction at 0.3–0.8″ scales.
• Representative continuum fits for type-1 and type-2 nuclei: compare silicate 9.7/18 µm profiles and continuum slopes, and see how the flared-disk (clumpy+smooth, variable grain size) reproduces them versus alternatives.
• Fit-quality and model-preference summary: histogram/scatter of chi2/dof and best-fit families across the 21 targets, confirming 12/21 successes, two disk+wind, one clumpy torus, and the 40% failures.
• Stacked residuals in the 5–8 µm region: highlight systematic deficits at ~6.0 µm (H2O ice) and 6.85/7.25 µm (aliphatic hydrocarbons) that current models miss.
• Posterior constraints on dust grain size in successful flared-disk fits: inspect whether large grains are favored and any trend with optical type (1 vs 2).