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

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