Dust in Active Galactic Nuclei: Anomalous Silicate to Optical Extinction Ratios?

TL;DR

This study investigates the dust properties in active galactic nuclei (AGNs), revealing a lower optical-to-silicate extinction ratio than in the Milky Way, likely due to larger dust grains in the AGN torus.

Contribution

It provides the first systematic analysis of optical extinction and silicate absorption in a large sample of AGNs, highlighting differences from the Galactic interstellar medium.

Findings

A_V/9.7 ratio is about 5.5, lower than the Galactic value of 18.5.

AGNs likely have larger dust grains in their tori compared to the Milky Way.

The dust grain size distribution affects the extinction properties in AGNs.

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 (\Delta\tau9.7) of 110 type 2 AGNs. We derive A_V from the Balmer decrement based on the Sloan Digital Sky Survey data, and \Delta\tau9.7 from the Spitzer/Infrared Spectrograph data. We find that with a mean ratio of A_V/\Delta\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/\Delta\tau9.7 ~ 18.5. We argue that the anomalously low A_V/\Delta\tau9.7 ratio could be due to the predominance of larger grains in the AGN torus compared to that in the Galactic diffuse ISM.