Letters to the Editor

The XMM Deep survey in the CDF-S

I. First results on heavily obscured AGN^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: andrea.comastri@oabo.inaf.it
² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ ICREA and Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franqués 1, 08028 Barcelona, Spain
⁴ Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
⁵ Department of Astronomy and Astrophysics, 525 Davey Lab, the Pennsylvania State University, University Park, PA 16802, USA
⁶ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁸ University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹¹ Max-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, 85748 Garching bei München, Germany
¹² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹³ Dipartimento di Scienze Fisiche, Università di Napoli “Federico II”,V. Cinthia 9, 80126 Napoli, Italy
¹⁴ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
¹⁵ Institute for the Physics and Mathematics of the Universe (IPMU), IPMU, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa Chiba 277-8568, Japan
¹⁶ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁷ Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile

Received: 10 November 2010
Accepted: 16 December 2010

Abstract

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 N_H ≈ 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.