The Origin of the Silicate Emission Features in the Seyfert 2 Galaxy, NGC 2110

TL;DR

This study investigates the origin of silicate emission features in the Seyfert 2 galaxy NGC 2110, constraining the emission region to within 32 pc of the nucleus and exploring models of the torus and narrow line region as potential sources.

Contribution

It provides the strongest constraint yet on the size of the silicate emitting region in a Seyfert galaxy and compares observational data with clumpy torus models to identify possible emission origins.

Findings

Silicate emission region is within 32 pc of the nucleus.

Edge-on torus models can explain the silicate features.

Small number of clouds in the torus models fit the data well.

Abstract

The unified model of active galactic nuclei (AGN) predicts silicate emission features at 10 and 18 microns in type 1 AGN, and such features have now been observed in objects ranging from distant QSOs to nearby LINERs. More surprising, however, is the detection of silicate emission in a few type 2 AGN. By combining Gemini and Spitzer mid-infrared imaging and spectroscopy of NGC 2110, the closest known Seyfert 2 galaxy with silicate emission features, we can constrain the location of the silicate emitting region to within 32 pc of the nucleus. This is the strongest constraint yet on the size of the silicate emitting region in a Seyfert galaxy of any type. While this result is consistent with a narrow line region origin for the emission, comparison with clumpy torus models demonstrates that emission from an edge-on torus can also explain the silicate emission features and 2-20 micron spectral energy distribution of this object. In many of the best-fitting models the torus has only a small number of clouds along the line of sight, and does not extend far above the equatorial plane. Extended silicate-emitting regions may well be present in AGN, but this work establishes that emission from the torus itself is also a viable option for the origin of silicate emission features in active galaxies of both type 1 and type 2.