2010MNRAS.406L...6K

The first detection of the silicate absorption feature in Active Galactic Nuclei (AGNs) was made at 9.7µm for the prototypical Seyfert 2 galaxy NGC1068 35 yr ago, indicating the presence of a large column of silicate dust in the line of sight to the nucleus. It is now well recognized that type 2 AGNs exhibit prominent silicate absorption bands, while the silicate bands of type 1 AGNs appear in emission. More recently, using the Mid-Infrared Interferometric Instrument on the Very Large Telescope Interferometer, Jaffe et al. for the first time spatially resolved the parsec-sized dust torus around NGC1068 and found that the 10µm silicate absorption feature of the innermost hot component exhibits an anomalous profile differing from that of the interstellar medium and that of common olivine-type silicate dust. While they ascribed the anomalous absorption profile to gehlenite (Ca₂Al₂SiO₇, a calcium aluminium silicate species), we propose a physical dust model and argue that, although the presence of gehlenite is not ruled out, the anomalous absorption feature mainly arises from silicon carbide.