On the Anomalous Silicate Absorption Feature of the Prototypical Seyfert 2 Galaxy NGC 1068

TL;DR

This paper investigates the unusual silicate absorption feature in NGC 1068, proposing that silicon carbide, rather than gehlenite, primarily causes the anomaly observed in the dust around this Seyfert 2 galaxy.

Contribution

It introduces a new physical dust model suggesting silicon carbide as the main cause of the anomalous silicate absorption profile in NGC 1068.

Findings

Silicon carbide can explain the anomalous absorption feature.

Gehlenite presence is not ruled out but is less likely the main cause.

The model aligns with spatially resolved mid-infrared observations.

Abstract

The first detection of the silicate absorption feature in AGNs was made at 9.7 micrometer for the prototypical Seyfert 2 galaxy NGC 1068 over 30 years 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. (2004) by the first time spatially resolved the parsec-sized dust torus around NGC 1068 and found that the 10 micrometer 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 (Ca_2Al_2SiO_7, a calcium aluminum 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.