Infrared Signatures of Dusty Tori Formed by MHD-Driven Outflows

Abstract

We aim to probe the physical structure and conditions of the central regions of active galactic nuclei (AGN) by interpreting their mid-infrared observational spectra. We constructed a sophisticated three-dimensional radiative transfer model of the AGN source and dusty torus, built upon a physically motivated framework that includes a comprehensive dust grain model. To investigate the properties of the circumnuclear medium, we performed model fitting to a sample of 20 observational spectra from Spitzer Space Telescope's IRS Enhanced Data Products, testing two distinct dust density laws: a steep $r^{-2}$ profile, characteristic of a radiation pressure-driven wind, and a shallower $r^{-1}$ profile, which is a key prediction of magnetohydrodynamic (MHD) models. Our model successfully reproduces fundamental observational phenomena, such as the transition from 9.7\,$μ$m silicate emission at low inclinations to deep absorption at high inclinations, with absorption features becoming more pronounced for higher accretion rates, dust-to-gas ratios, and black hole masses. Most importantly, a direct comparison of the fitting results revealed that the $r^{-1}$ density law provided a significantly better fit ($χ^2$) than the $r^{-2}$ density law for 18 of the 20 observational spectra. These results confirm the physical consistency of our advanced radiative model and provide compelling evidence that an $r^{-1}$ density law is a more accurate representation of the AGN dusty torus.

Figures (12)

• Figure 1: Simplified 2D schematic of an AGN, illustrating key structural components including the central black hole (black), surrounding hot corona (orange), accretion disk (yellow), and obscuring dusty torus (gray). Blue lines represent the outflowing winds launched from the accretion disk, and $r(\theta)$ is the inner radius of the dusty torus with an inclination angle $\theta$.
• Figure 2: Left panel: spectral energy distribution of the central AGN source at an inclination angle of $\theta = 75^\circ$. The spectral shape, characterized by $\alpha_{\mathrm{ox}} = -1.5$, remains consistent across all viewing angles Sarangi_2019. The surrounding material is entirely heated by the central AGN radiation. right panel: angle-dependent luminosity function, following the luminosity profile defined in Eq. (\ref{['L']}), showing how the observed emission varies with inclination angle due to anisotropic radiation effects.
• Figure 3: Mass absorption coefficient $\kappa_{abs}$ as a function of wavelength for different grain sizes, $0.01 \mu m$ (blue), $0.1 \mu m$ (yellow), $1 \mu m$ (green) and a mixed population (black), including contributions from Astrodust and PAH components Hensley_2023.
• Figure 4: Rest-frame mid-infrared starburst templates used in this work. Normalized $F_{\lambda}$ spectra of Arp 220 (blue), M 82 (orange), and NGC 253 (green) are shown as a function of $\lambda_{\rm rest}$. Prominent PAH complexes at 6.2, 7.7, 8.6, 11.3, 12.7, and 17$\mu$m and the silicate absorption features at 9.7 and 18 $\mu$m are indicated by vertical dashed lines.
• Figure 5: Polar diagrams illustrating the spatial distribution of AGN dusty torus under varying black hole masses and accretion rates. The panels in the first row correspond to an accretion rate of $\dot{m}=0.1$, while those in the second row correspond to $\dot{m}=1$. From left to right, the black hole mass increases from $10^6 M_{\odot}$ to $10^8 M_{\odot}$. The radial extent and angular coverage of the dust (purple shading) are shown, overlaid with the sublimation radius at 1500 K ($R_{1500 K}$, blue line) and the outer boundary of the torus ($R_{inf}$, orange line).