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 which forms a torus around the AGN. In this Letter we report a systematic exploration of the optical extinction (A_V) and the silicate absorption optical depth (Δ\tau9.7) of 110 type 2 AGNs. We derive A_V from the Balmer decrement based on the Sloan Digital Sky Survey data, and Δ\tau9.7 from the Spitzer/Infrared Spectrograph data. We find that with a mean ratio of A_V/Δ\tau9.7 ~ 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 A_V/Δ\tau9.7 ~ 18.5. We argue that the anomalously low A_V/Δ\tau9.7 ratio could be due to the predominance of larger grains in the AGN torus compared to that in the Galactic diffuse ISM.

ApJL, 792, L9, in print

References in corpus (4)

• AGN Dusty Tori: I. Handling of Clumpy Media
• Mid-IR Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width
• The innermost region of AGN tori: implications from the HST/NICMOS Type 1 point sources and near-IR reverberation
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• The Past and Future of Mid-Infrared Studies of AGN
• Modelling the strongest silicate emission features of local type 1 AGN
• The complex infrared dust continuum emission of NGC1068: ground-based N- and Q-band spectroscopy and new radiative transfer models