Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 \mu m , that have optical depths of \tau > 1 .
Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei ( hereafter AGN ) , we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN .
We compile X-ray spectroscopic observations available in the literature on the optically-thick ( \tau _ { 9.7 \mu m } > 1 ) sources from the 12 \mu m IRAS Seyfert sample .
We find that the majority of the high- \tau optically confirmed Seyferts ( six out of nine ) in the 12 \mu m sample are probably Compton-thick .
Thus , we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN , with the success rate being close to 70 % in the local Universe .
This is at least comparable to , if not better than , other rates obtained with photometric information in the mid to far-IR , or even mid-IR to X-rays .
However , this technique can not provide complete Compton-thick AGN samples , i.e. , there are many Compton-thick AGN that do not display significant Si absorption , with the most notable example being NGC 1068 .
After assessing the validity of the high 9.7 \mu m optical-depth technique in the local Universe , we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts .
We compile a sample of seven high- \tau Spitzer sources in the Great Observatories Origins Deep Survey ( GOODS ) and five in the Spitzer First-Look Survey .
All these have been selected to have no PAH features ( EW _ { 6.2 \mu m } < 0.3 \mu m  ) to maximise the probability that they are bona-fide AGN .
Six out of the seven GOODS sources have been detected in X-rays , while for the five FLS sources only X-ray flux upper limits are available .
The high X-ray luminosities ( L _ { X } > 10 ^ { 42 } erg s ^ { -1 } Â ) of the detected GOODS sources corroborates that these are AGN .
For FLS , ancillary optical spectroscopy reveals hidden nuclei in two more sources .
SED fitting can support the presence of an AGN in the vast majority of sources .
Owing to the limited photon statistics , we can not derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick .
However , the low L _ { X } / L _ { 6 \mu m } luminosity ratios , suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN .