Infrared Signatures of Dusty Tori Formed by MHD-Driven Outflows

Ruiyu Pan, Arkaprabha Sarangi

First listed 2025-11-14 | Last updated 2026-03-15

Abstract

We investigate the radial structure of active galactic nuclei (AGN) dusty tori by forward-modeling mid-infrared (MIR) spectra observed with JWST/MIRI-MRS for a sample of 25 AGN. We develop a physically motivated three dimensional radiative transfer framework to test various magnetohydrodynamic (MHD) wind scenarios characterized by radial density laws of the form $n(r)\propto r^{-p}$ for $p \in \{0.5, 1.0, 1.5, 2.0\}$. Our Bayesian analysis reveals a pronounced structural dichotomy rather than a single universal density law: a majority (14/25) of the sources favor the flattest profile ($p=0.5$), while a significant minority (8/25) statistically prefers the steepest, most compact distribution ($p=2.0$). We also identify a systematic redward shift in the silicate emission peak, consistent with the presence of micron-sized, processed grains in the inner regions of the wind. Critically, we find that the inferred AGN fractional contribution, $f_{\rm AGN}$, is highly sensitive to the assumed radial index, exhibiting up to a four-fold systematic shift between $p=0.5$ and $p=2.0$. This p-degeneracy highlights that static structural assumptions can significantly bias AGN power estimates. The identified structural diversity suggests that MHD-driven winds may exist in multiple dynamical states, though the precise physical drivers governing these configurations remain to be uniquely determined. These results underscore the limitations of single-profile models and emphasize the need for high-resolution spatial constraints to break these fundamental degeneracies.

Short digest

The authors forward-model mid-IR spectra for 25 AGN with a 3D radiative-transfer framework that tests MHD-wind density laws n(r)∝r^{-p} across p=0.5–2.0 while decomposing host starburst emission. They find a structural dichotomy: most sources (14/25) favor a very shallow profile (p=0.5), with a substantial minority (8/25) preferring a compact distribution (p=2.0), and they report a systematic redward shift of the silicate emission peak indicative of micron-sized, processed grains. Crucially, the inferred AGN fraction f_AGN swings by up to a factor of four with p, showing that static assumptions can strongly bias power estimates. The work argues that high-resolution spatial constraints are needed to break these degeneracies and pin down the drivers of multiple MHD wind states.

Key figures to inspect

• Posterior on the radial index p (0.5–2.0) for the 25-source sample—inspect Bayes factors or model probabilities that reveal the split between flat (p=0.5) and compact (p=2.0) tori.
• Representative MIRI-MRS spectra with best-fit models for a ‘flat’ (p=0.5) and a ‘compact’ (p=2.0) target—compare the 9.7 μm silicate profile and the redward peak shift that signals micron-sized grains.
• f_AGN versus assumed p for individual objects—quantify the up-to-4× swing and identify sources most sensitive to the radial-law choice.
• Inclination grid from the radiative-transfer model—trace the transition from silicate emission to deep absorption and how depth scales with accretion rate, dust-to-gas ratio, and black-hole mass.
• AGN+starburst SED decomposition panel—verify PAH/cool-continuum removal before torus fitting and its impact on recovered silicate strength and f_AGN.