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Accretion onto SMBH is believed to occur mostly in obscured AGN. Such objectsare proving rather elusive in surveys of distant galaxies, including those atX-ray energies. Our main goal is to determine whether the revised IRAC criteriaof Donley et al. (2012) (objects with an IR power-law spectral shape), areeffective at selecting X-ray type-2 AGN. We present the results from the X-rayspectral analysis of 147 AGN selected by cross-correlating the highest spectralquality 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 orderto measure the amount of intrinsic absorption in these sources, we adopt asimple X-ray spectral model that includes a power-law modified by intrinsicabsorption and a possible soft X-ray component. We find 21/147 sources to beheavily absorbed but the uncertainties in their obscuring column densities donot allow us to confirm their Compton-Thick nature without resorting toadditional criteria. Although IR power-law galaxies are less numerous in oursample than IR non-power-law galaxies (60 versus 87 respectively), we find thatthe fraction of absorbed (N_{H} > 10^{22} cm^{-2}) AGN is significantly higher(at about 3 sigma level) for IR-power-law sources ($\sim$2/3) than for thosesources that do not meet this IR selection criteria ($\sim$1/2). This behaviouris particularly notable at low luminosities, but it appears to be present,although with a marginal significance, at all luminosities. We thereforeconclude that the IR power-law method is efficient in finding X-ray-absorbedsources. We would then expect that the long-sought dominant population ofabsorbed AGN is abundant among IR power-law spectral shape sources not detectedin X-rays.

The XMM-Newton deep survey in the CDF-S VI. Obscured AGN selected as infrared power-law galaxies