Silicate Dust in Active Galactic Nuclei

TL;DR

This study analyzes silicate dust properties in 147 AGNs, revealing that the dust is predominantly micron-sized, differs from Galactic dust, and influences the observed infrared emission features, supporting the AGN unification model.

Contribution

It provides detailed modeling of silicate emission spectra in AGNs, showing that dust grains are larger and compositionally diverse compared to Galactic dust, with implications for AGN dust extinction laws.

Findings

Most AGN silicate dust is micron-sized, larger than Galactic grains.

The dust composition varies, with some fitting astronomical silicate and others amorphous olivine or pyroxene.

The 9.7 μm emission mainly comes from warm silicate dust at ~270 K.

Abstract

The unification theory of active galactic nuclei (AGNs) hypothesizes that all AGNs are surrounded by an anisotropic dust torus and are essentially the same objects but viewed from different angles. However, little is known about the dust which plays a central role in the unification theory. There are suggestions that the AGN dust extinction law appreciably differs from that of the Galaxy. Also, the silicate emission features observed in type 1 AGNs appear anomalous (i.e., their peak wavelengths and widths differ considerably from that of the Galaxy). In this work, we explore the dust properties of 147 AGNs of various types at redshifts z<0.5, with special attention paid to 93 AGNs which exhibit the 9.7 and 18 $\mu$m silicate emission features. We model their silicate emission spectra obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. We find that 60/93 of the observed spectra can be well explained with "astronomical silicate", while the remaining sources favor amorphous olivine or pyroxene. Most notably, all sources require the dust to be $\mu$m-sized (with a typical size of ~1.5$\pm$0.1 $\mu$m), much larger than sub-$\mu$m-sized Galactic interstellar grains, implying a flat or "gray" extinction law for AGNs. We also find that, while the 9.7 $\mu$m emission feature arises predominantly from warm silicate dust of temperature T~270 K, the ~5--8 $\mu$m continuum emission is mostly from carbon dust of T~640 K. Finally, the correlations between the dust properties (e.g., mass, temperature) and the AGN properties (e.g., luminosity, black hole mass) have also been investigated.