Spitzer/IRS full spectral modeling to characterize mineralogical properties of silicate dust in heavily obscured AGNs

TL;DR

This study models the full mid-infrared spectra of heavily obscured AGNs to analyze silicate dust properties, revealing insights into dust composition, crystallinity, and the influence of AGN activity.

Contribution

It extends previous work by modeling a wider spectral range with radiative transfer, comparing dust models, and providing a more reliable assessment of dust mineralogy in obscured AGNs.

Findings

Porous silicate dust models are preferred in most cases.

Silicate dust in AGNs shows high crystallinity and small grain sizes.

Dust properties suggest recent starburst activity and AGN-induced processing.

Abstract

Mid-Infrared (IR) silicate dust bands observed in heavily obscured active galactic nuclei (AGNs) include information on the mineralogical properties of silicate dust. We aim to investigate the mineralogical picture of the circumnuclear region of heavily obscured AGNs to reveal obscured AGN activities through the picture. In our previous study Tsuchikawa et al. (2021), we investigated the properties of silicate dust in heavily obscured AGNs focusing on the mineralogical composition and the crystallinity with Spitzer/IRS 5.3-12 micron spectra. In this study, we model the full-range Spitzer/IRS 5-30 micron spectra of 98 heavily obscured AGNs using a one-dimensional radiative transfer calculation with four dust species in order to evaluate wider ranges of the properties of silicate dust more reliably. Comparing fitting results between four dust models with a different size and porosity, 95 out of the 98 galaxies prefer a porous silicate dust model without micron-sized large grains. The pyroxene mass fraction and the crystallinity are overall consistent with but significantly different from the previous results for the individual galaxies. The pyroxene-poor composition, small dust size and high porosity are similar to newly formed dust around mass-loss stars as seen in our Galaxy, which presumably originates from the recent circumnuclear starburst activity. The high crystallinity on average suggests dust processing induced by AGN activities.