DUST IN ACTIVE GALACTIC NUCLEI: ANOMALOUS SILICATE TO OPTICAL EXTINCTION RATIOS?

摘要

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 (A_V) and the silicate absorption optical depth (Δτ_(9.7)) of 110 type 2 AGNs. We derive AV from the Balmer decrement based on the Sloan Digital Sky Survey data, and Δτ_(9.7) from the Spitzer/InfraRed Spectrograph data. We find that with a mean ratio of ≤ 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 /Δτ_(9.7) ≈ 18.5. We argue that the anomalously low A_V /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.