The XMM-Newton deep survey in the CDF-S VI. Obscured AGN selected as
  infrared power-law galaxies
  Accretion onto SMBH is believed to occur mostly in obscured AGN. Such objects
are proving rather elusive in surveys of distant galaxies, including those at
X-ray energies. Our main goal is to determine whether the revised IRAC criteria
of Donley et al. (2012) (objects with an IR power-law spectral shape), are
effective at selecting X-ray type-2 AGN. We present the results from the X-ray
spectral analysis of 147 AGN selected by cross-correlating the highest spectral
quality ultra-deep XMM-Newton and the Spitzer/IRAC catalogues in the CDF-S.
Consequently it is biased towards sources with high S/N X-ray spectra. In order
to measure the amount of intrinsic absorption in these sources, we adopt a
simple X-ray spectral model that includes a power-law modified by intrinsic
absorption and a possible soft X-ray component. We find 21/147 sources to be
heavily absorbed but the uncertainties in their obscuring column densities do
not allow us to confirm their Compton-Thick nature without resorting to
additional criteria. Although IR power-law galaxies are less numerous in our
sample than IR non-power-law galaxies (60 versus 87 respectively), we find that
the fraction of absorbed (N_{H} > 10^{22} cm^{-2}) AGN is significantly higher
(at about 3 sigma level) for IR-power-law sources ($\sim$2/3) than for those
sources that do not meet this IR selection criteria ($\sim$1/2). This behaviour
is particularly notable at low luminosities, but it appears to be present,
although with a marginal significance, at all luminosities. We therefore
conclude that the IR power-law method is efficient in finding X-ray-absorbed
sources. We would then expect that the long-sought dominant population of
absorbed AGN is abundant among IR power-law spectral shape sources not detected
in X-rays.
