  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.

Cosmology and Nongalactic Astrophysics