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.
|Keywords:||Galaxies, Active, Seyfert, Infrared, X-rays.|
|Full text:||(VoR) Version of Record|
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|Publisher Web site:||http://dx.doi.org/10.1088/0004-637X/755/1/5|
|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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