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

The unified model of active galactic nuclei (AGNs) predicts silicate emission features at 10 and 18 μm in type 1 AGNs, and such features have now been observed in objects ranging from distant quasi-stellar objects to nearby LINERs. More surprising, however, is the detection of silicate emission in a few type 2 AGNs. 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 μm 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 AGNs, 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.