Modelling the strongest silicate emission features of local type 1 AGN

TL;DR

This study measures and models the 10 and 18 micron silicate emission features in local type 1 AGN, finding that clumpy torus models with outflows best reproduce observed spectra, highlighting complex dust and geometric effects.

Contribution

It provides detailed measurements of silicate features in local AGN and compares multiple models, identifying the most accurate geometries and dust compositions for these emissions.

Findings

Silicate features peak at 10.3 and 17.3 microns with specific strengths.

Clumpy torus models with outflows best fit the observed spectra.

Stellar contributions are needed at short wavelengths even in AGN-dominated galaxies.

Abstract

We measure the 10 and $18\mu$m silicate features in a sample of 67 local ($z<0.1$) type 1 active galactic nuclei (AGN) with available {\it Spitzer} spectra dominated by non-stellar processes. We find that the $10\mu$m silicate feature peaks at $10.3^{+0.7}_{-0.9}\mu$m with a strength (Si$_{p}$ = ln f$_{p}$(spectrum)/f$_{p}$(continuum)) of $0.11^{+0.15}_{-0.36}$, while the $18\mu$m one peaks at $17.3^{+0.4}_{-0.7}\mu$m with a strength of $0.14^{+0.06}_{-0.06}$. We select from this sample sources with the strongest 10$\mu$m silicate strength ($\sigma_{Si_{10\mu m}}>0.28$, 10 objects). We carry out a detailed modeling of the IRS/{\it Spitzer} spectra by comparing several models that assume different geometries and dust composition: a smooth torus model, two clumpy torus models, a two-phase medium torus model, and a disk+outflow clumpy model. We find that the silicate features are well modeled by the clumpy model of Nenkova et al. 2008, and among all models those including outflows and complex dust composition are the best (Hoenig et al. 2017). We note that even in AGN-dominated galaxies it is usually necessary to add stellar contributions to reproduce the emission at the shortest wavelengths.