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To optimally identify candidates of the Compton-thick (CT) active galacticnuclei (AGN) that contribute to the unresolved X-ray background in infraredsurveys, a tracer of column density is desirable in addition to an AGNindicator. In a recent study, we aimed to test whether the 9.7 micron silicateabsorption feature can be used for this purpose when seen at high opticaldepths. We found that the extreme criterion of optical thickness at 9.7 micronis efficient in identifying CT objects among local AGN. Having identified sixof the nine CT AGN in the 12 micron sample with Spitzer and X-ray spectra, weexpanded this analysis at intermediate/high z, using all GOODS and FLS sourceswith Spitzer and X-ray observations. We found 12 sources with tau_9.7>1 thathost an AGN between 0.8<z<2.7. Four of them are likely to be CT according totheir low X-ray to 6 micron luminosity ratio. Surveys with complete coverage inboth mid-infrared spectra and X-ray data can provide large populations of suchsources, as at least 5-9% of all infrared bright galaxies in the GOODS and FLSsamples are tau_9.7>1 AGN.

Querying for heavily obscured AGN via high 9.7 micron optical depths: results from the 12 micron, GOODS, and FLS Spitzer spectroscopic samples