We present the first results of the spectroscopy of distant, obscured AGN as obtained with the ultra-deep (≈3.3 Ms) XMM-Newton survey in the Chandra deep field South. One of the primary goals of the project is to characterize the X-ray spectral properties of obscured and heavily obscured Compton-thick AGN over the range of redhifts and luminosities that are relevant in terms of their contribution to the X-ray background. The ultra-deep exposure, coupled with the XMM detector’s spectral throughput, allowed us to accumulate good quality X-ray spectra for a large number of X-ray sources and, in particular, for heavily obscured AGN at cosmological redshifts. Specifically we present the X-ray spectral properties of two high-redshift – z = 1.53 and z = 3.70 – sources. The XMM spectra of both are very hard, with a strong iron Kα line at a rest-frame energy of ~6.4 keV. A reflection-dominated continuum provides the best description of the X-ray spectrum of the z = 1.53 source, while the intrinsic continuum of the z = 3.70 AGN is obscured by a large column NH ≈ 1024 cm-2 of cold gas. Compton-thick absorption, or close to it, is unambiguously detected in both sources. Interestingly, these sources would not be selected as candidate Compton thick AGN by some multiwavelength selection criteria based on the mid-infrared to optical and X-ray to optical flux ratios.

Support from the Italian Space Agency (ASI) under the contracts ASI-INAF I/088/06/0 and I/009/10/0 is acknowledged. A.C. acknowledges the NASA grant NNX09AQ05G to visit CfA, where part of this work was written. I.G. and A.C. acknowledge the Marie Curie fellowship FP7-PEOPLE-IEF-2008 Prop. 235285. X.B. and F.J.C. acknowledge financial support from the Spanish Ministerio de Ciencia e Innovación under project AYA2009-08059. M.B. acknowledges support by the Deutsches Zentrum für Luft- und Raumfahrt, DLR project numbers 50 OR 0207 and 50 OR 0405. F.C., F.N. and M.B. acknowledge NASA grants NNX08AX51G and NNX09AQ05G. F.C. acknowledges support from the Blanceflor Boncompagni Ludovisi Foundation. J.D.S. is supported by World Premier International (WPI) Research Center Initiative , MEXT, Japan. W.N.B. acknowledges NASA grant SV0-80004 and NASA ADP grant NNX10AC99G. F.E.B. cknowledges the support of CONICYT, Chile, under grants FONDECYT 1101024 and FONDAP (CATA) 15010003.

Peer reviewed