On the Anomalous Silicate Emission Features of AGNs: A Possible Interpretation Based on Porous Dust

TL;DR

This paper proposes that the anomalous silicate emission features observed in AGNs can be explained by porous composite dust, which differs from typical interstellar medium dust, due to grain coagulation in dense regions.

Contribution

It introduces a model of porous composite dust to explain the shifted and broadened silicate emission features in AGNs, offering a new interpretation of dust properties.

Findings

Porous dust explains the peak shift and broadening of silicate features.

Porous dust composition includes amorphous silicate, amorphous carbon, and vacuum.

Grain coagulation leads to porous dust in AGN environments.

Abstract

The recent Spitzer detections of the 9.7 micron Si--O silicate emission in type 1 AGNs provide support for the AGN unification scheme. The properties of the silicate dust are of key importance to understanding the physical, chemical and evolutionary properties of the obscuring dusty torus around AGNs. Compared to that of the Galactic interstellar medium (ISM), the 10 micron silicate emission profile of type 1 AGNs is broadened and has a clear shift of peak position to longer wavelengths. In literature this is generally interpreted as an indication of the deviations of the silicate composition, size, and degree of crystallization of AGNs from that of the Galactic ISM. In this Letter we show that the observed peak shift and profile broadening of the 9.7 micron silicate emission feature can be explained in terms of porous composite dust consisting of ordinary interstellar amorphous silicate, amorphous carbon and vacuum. Porous dust is naturally expected in the dense circumnuclear region around AGNs, as a consequence of grain coagulation.