On the Optical-to-Silicate Extinction Ratio as a Probe of the Dust Size in Active Galactic Nuclei

Abstract

Dust plays a central role in the unification theory of active galactic nuclei (AGNs). Whether the dust that forms the torus around an AGN is tenth-$\mu$m-sized like interstellar grains or much larger has a profound impact on correcting for the obscuration of the dust torus to recover the intrinsic spectrum and luminosity of the AGN. Here we show that the ratio of the optical extinction in the visual band ($A_V$) to the optical depth of the 9.7 $\mu$m silicate absorption feature ($\Delta {\tau }_{9.7}$) could potentially be an effective probe of the dust size. The anomalously lower ratio of $A_V/\Delta {\tau }_{9.7}\approx 5.5$ of AGNs compared to that of the Galactic diffuse interstellar medium of $A_V/\Delta {\tau }_{9.7}\approx 18$ reveals that the dust in AGN torus could be substantially larger than the interstellar grains of the Milky Way and of the Small Magellanic Cloud, and therefore, one might expect a flat extinction curve for AGNs.

20 pages, 9 figures; accepted for publication in The Astrophysical Journal

References in corpus (6)

• Dust in starburst nuclei and ULIRGs: SED models for observers
• On the Crystallinity of Silicate Dust in the Interstellar Medium
• Silicate Dust in Active Galactic Nuclei
• Modeling the Infrared Interstellar Extinction
• Dust in Active Galactic Nuclei: Anomalous Silicate to Optical Extinction Ratios?
• Mid-infrared extinction and fresh silicate dust towards the Galactic Center

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• The Past and Future of Mid-Infrared Studies of AGN
• Active Galactic Nuclei as seen by the Spitzer Space Telescope
• The Subarcsecond Mid-Infrared View of Local Active Galactic Nuclei. IV. The L- and M-band Imaging Atlas
• Comparative analysis of the influence of Sgr A* and nearby active galactic nuclei on the mass loss of known exoplanets
• Dissecting the active galactic nucleus in Circinus -- III. VLT/FORS2 polarimetry confirms dusty cone illuminated by a tilted accretion disc
• Probing the 9.7μm Interstellar Silicate Extinction Profile through the Spitzer/IRS Spectroscopy of OB Stars
• The role of grain size in AGN torus dust models
• The complex infrared dust continuum emission of NGC1068: ground-based N- and Q-band spectroscopy and new radiative transfer models
• Cosmic Stillness: High Quiescent Galaxy Fractions Across Upper Mass Scales in the Early Universe to z = 7 with JWST
• Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations