Dust in active galactic nuclei: Anomalous silicate to optical extinction ratios?

Abstract

Dust plays a central role in the unification theory of active galactic nuclei (AGNs). However, little is known about the nature (e.g., size, composition) of the dust that forms a torus around the AGN. In this Letter, we report a systematic exploration of the optical extinction (AV) and the silicate absorption optical depth (Δτ9.7) of 110 type 2 AGNs. We derive AV from the Balmer decrement based on the Sloan Digital Sky Survey data, and Δτ9.7 from the Spitzer/InfraRed Spectrograph data. We find that with a mean ratio of 〈AV /Δτ9.7〉 ≲ 5.5, the optical-to-silicate extinction ratios of these AGNs are substantially lower than that of the Galactic diffuse interstellar medium (ISM) for which AV /Δτ9.7 18.5. We argue that the anomalously low AV /Δτ 9.7 ratio could be due to the predominance of larger grains in the AGN torus compared to that in the Galactic diffuse ISM. © 2014. The American Astronomical Society. All rights reserved..