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

Goulding, A.D.; Alexander, D.M.; Bauer, F.E.; Forman, W.R.; Hickox, R.C.; Jones, C.; Mullaney, J.R.; Trichas, M.

Abstract

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.

Citation

Goulding, A., Alexander, D., Bauer, F., Forman, W., Hickox, R., Jones, C., …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), Article 5. https://doi.org/10.1088/0004-637x/755/1/5

• Journal Article Type: Article
• Publication Date: Aug 1, 2012
• Deposit Date: Apr 8, 2013
• Publicly Available Date: Dec 23, 2013
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 755
• Issue: 1
• Article Number: 5
• DOI: https://doi.org/10.1088/0004-637x/755/1/5
• Keywords: Galaxies, Active, Seyfert, Infrared, X-rays.
• Public URL: https://durham-repository.worktribe.com/output/1457241