X-ray observations of highly obscured 9.7 micron sources: an efficient
  method for selecting Compton-thick AGN ?
  Spitzer/IRS has revealed many sources with very deep Si features at 9.7micron
(tau>1). We set out to investigate whether a strong Si absorption feature is a
good indicator for the presence of a heavily obscured AGN. We compile X-ray
spectroscopic observations available in the literature on the
optically-thick,tau(9.7)>1 sources from the IRAS Seyfert sample. We find that
the majority of the high-tau optically confirmed Seyferts (6/9) in this sample
are probably CT. Thus we provide direct evidence for a connection between
mid-IR optically-thick galaxies and CT AGN, with the success rate being close
to 70% in the local Universe. This is at least comparable, if not better, than
other rates obtained with photometric information in the mid to far-IR, or even
mid-IR to Xray. However, this technique cannot provide complete CT AGN
samples,ie there are many CT AGN which do not show significant Si absorption,
with the most notable example being N1068. Having assessed the validity of the
high 9.7micron technique locally, we attempt to construct a sample of candidate
CT AGN at higher redshifts. We compile a sample of 7 high-tau sources in the
GOODS and 5 in the Spitzer FLS. All these have been selected to have no PAH
features EW(6.2)<0.3 in order to maximize the probability that they are AGN. 6
out of 7 sources in the GOODS 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 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.
We cannot derive useful X-ray spectroscopy constraints on whether these are CT.
However, the low LX/L6 ratios, suggest that at least 4 out of the 6 detected
sources in GOODS may be associated with CT AGN.
