Castelló Mor, N.Carrera, F. J.Alonso Herrero, A.Mateos, S.Barcons, X.Ranalli, P.Pérez González, Pablo GuillermoComastri, A.Vignali, C.Georgantopoulos, I.2023-06-192023-06-192013-080004-636110.1051/0004-6361/201321457https://hdl.handle.net/20.500.14352/35208© ESO, 2013. We are grateful to the referee for comments that helped improve the paper. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. N.C.-M., F.J.C., S.M. and X.B. acknowledge financial support provided by the Spanish Ministry of Economy and Competitiveness through grant AYA2010-21490-C02-01. S.M., F.J.C. and A.A.-H. acknowledge financial support by the Spanish Ministry of Economy and Competitiveness through grants AYA2010-21490-C02-01 and AYA2012-31447. SM acknowledges financial support from the JAE-Doc program (Consejo Superior de Investigaciones Científicas, cofunded by FSE). A.A.-H. acknowledges support from the Universidad de Cantabria through the Augusto G. Linares program. P.G.P.-G. acknowledges support from the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA2009-07723-E and AYA2009-10368. This work has made use of the Rainbow Cosmological Surveys Database, which is operated by the Universidad Complutense de Madrid (UCM). We acknowledge financial contribution from the agreement ASI-INAF I/009/10/0 and from the INAF-PRIN-2011.Context. Accretion onto supermassive black holes is believed to occur mostly in obscured active galactic nuclei (AGN). Such objects are proving rather elusive in surveys of distant galaxies, including those at X-ray energies. Aims. Our main goal is to determine whether the revised IRAC criteria of Donley et al. (2012, ApJ, 748, 142; objects with an infrared (IR) power-law spectral shape), are effective at selecting X-ray type-2 AGN (i.e., absorbed N_H > 10^22 cm^-2). Methods. 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 Chandra Deep Field South. 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 at the redshift of each source and a possible soft X-ray component. Results. 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^intr > 10^22 cm^-2) AGN is significantly higher (at about 3 sigma level) for IR-power-law sources (similar to 2/3) than for those sources that do not meet this IR selection criteria (~1/2). This behaviour is particularly notable at low luminosities, but it appears to be present, although with a marginal significance, at all luminosities. Conclusions. 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.engjournal articlehttp://dx.doi.org/10.1051/0004-6361/201321457http://www.aanda.org/open access52Active galactic nucleiSpitzer-space-telescopeSpectral energy-distributionQuasi-stellar objectsStar-forming galaxiesField-south surveyX-ray sourcesMidinfrared selectionSynthesis modelsSource catalogsAstrofísicaAstronomía (Física)