Extended silicate dust emission in PG QSOs

TL;DR

This study uses Spitzer mid-infrared spectra of 23 QSOs to show that silicate dust emission mainly originates from the extended narrow line region rather than the inner torus, challenging previous models.

Contribution

It provides evidence that silicate emission in QSOs comes from the extended NLR, not just the inner torus, based on spectral fitting and dust radius calculations.

Findings

Silicate emission radii are 100-200 times larger than sublimation radius.

Spectra fit well with dusty NLR models and black bodies.

Emission mainly from the innermost parts of the NLR.

Abstract

This paper addresses the origin of the silicate emission observed in PG QSOs, based on observations with the Spitzer Space Telescope. Scenarios based on the unified model suggest that silicate emission in AGN arises mainly from the illuminated faces of the clouds in the torus at temperatures near sublimation. However, detections of silicate emission in Type 2 QSOs, and the estimated cool dust temperatures, argue for a more extended emission region.To investigate this issue we present the mid-infrared spectra of 23 QSOs. These spectra, and especially the silicate emission features at ~10 and ~18 mu can be fitted using dusty narrow line region (NLR) models and a combination of black bodies. The bolometric luminosities of the QSOs allow us to derive the radial distances and covering factors for the silicate-emitting dust. The inferred radii are 100-200 times larger than the dust sublimation radius, much larger than the expected dimensions of the inner torus. Our QSO mid-IR spectra are consistent with the bulk of the silicate dust emission arising from the dust in the innermost parts of the NLR.