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Deep silicate absorption features in Compton-thick active galactic nuclei predominantly arise due to dust in the host galaxy.

Goulding, A.D. and Alexander, D.M. and Bauer, F.E. and Forman, W.R. and Hickox, R.C. and Jones, C. and Mullaney, J.R. and Trichas, M. (2012) 'Deep silicate absorption features in Compton-thick active galactic nuclei predominantly arise due to dust in the host galaxy.', Astrophysical journal., 755 (1). p. 5.


We explore the origin of mid-infrared (mid-IR) dust extinction in all 20 nearby (z < 0.05) bona fide Compton-thick (N H > 1.5 × 1024 cm-2) active galactic nuclei (AGNs) with hard energy (E > 10 keV) X-ray spectral measurements. We accurately measure the silicate absorption features at λ ~ 9.7 μm in archival low-resolution (R ~ 57-127) Spitzer Infrared Spectrograph spectroscopy, and show that only a minority (≈45%) of nearby Compton-thick AGNs have strong Si-absorption features (S 9.7 = ln (f int/f obs) >~ 0.5) which would indicate significant dust attenuation. The majority (≈60%) are star formation dominated (AGN:SB < 0.5) at mid-IR wavelengths and lack the spectral signatures of AGN activity at optical wavelengths, most likely because the AGN emission lines are optically extinguished. Those Compton-thick AGNs hosted in low-inclination-angle galaxies exhibit a narrow range in Si-absorption (S 9.7 ~ 0-0.3), which is consistent with that predicted by clumpy-torus models. However, on the basis of the IR spectra and additional lines of evidence, we conclude that the dominant contribution to the observed mid-IR dust extinction is dust located in the host galaxy (i.e., due to disturbed morphologies, dust lanes, galaxy inclination angles) and not necessarily a compact obscuring torus surrounding the central engine.

Item Type:Article
Keywords:Galaxies, Active, Seyfert, Infrared, X-rays.
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Publisher statement:© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
Date accepted:No date available
Date deposited:23 December 2013
Date of first online publication:August 2012
Date first made open access:No date available

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