Towards an observationally motivated AGN dusty torus model -- II. The roles of density distribution and chemical composition of the dust

TL;DR

This study investigates how dust density distribution and chemical composition affect the MIR emission in AGNs, revealing their significant impact on spectral features and the importance of diverse models for accurate representation.

Contribution

It introduces a comprehensive model grid exploring various dust distributions and compositions, analyzing their effects on AGN MIR spectra with radiative transfer simulations.

Findings

Dust distribution and composition significantly influence MIR spectral features.

Smooth distributions produce stronger silicate absorption; clumpy distributions favor emission features.

Large grains in ISM dust better reproduce observed MIR features, especially in Type-2 AGNs.

Abstract

Several models of nuclear dust in active galactic nuclei (AGN) have been presented in the literature to determine its physical and geometrical properties, usually assuming the dust density distribution as the main aspect producing differences in the mid-infrared (MIR) emission of AGNs. We present a study of the MIR emission of nearby AGNs by exploring the effects of dust distribution and chemical composition on the spectral energy distributions (SEDs) using radiative transfer simulations. Our model grid includes smooth, clumpy, and two-phase dust distributions, combined with two dust compositions: the interstellar medium (ISM) dust composition including large grains (up to $\rm{10 \ \mu m}$), and the oxide/silicate-based composition from Reyes-Amador et al. (2024). A synthetic SED library was generated and analysed both on a model-to-model basis and with observed MIR spectra from 68 AGNs. We found that dust density distribution and dust composition significantly influence the spectral shapes and silicate features at $10$ and $\rm{18 \ \mu m}$, especially at edge-on orientations. The smooth distribution produces stronger and broader silicate absorption features, while the clumpy distribution generates stronger features in emission. The two-phase distributions exhibit intermediate characteristics depending on the clumpiness fraction ($f_{\text{cl}}$) and filling factor ($f_{\text{fill}}$). The ISM dust composition with large grains is more suited to reproduce the observed features and a higher fraction of good fits, particularly with Type-2 SEDs, independently of dust density distributions. The Reyes-Amador et al. (2024) composition provides a larger number of good fits with Type-1 SEDs for $f_{\text{cl}} \leq 0.5$, and with Type-2 SEDs for $f_{\text{cl}} \geq 0.9$. This work shows that no single dust distribution or composition reproduces all observations.