2012ApJ...755....5G

We explore the origin of mid-infrared (mid-IR) dust extinction in all 20 nearby (z < 0.05) bona fide Compton-thick (N_H> 1.5x10²⁴/cm2) 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.