The XMM Deep survey in the CDFS I. First results on heavily obscured AGN

TL;DR

This study uses ultra-deep XMM-Newton observations to analyze the X-ray spectral properties of heavily obscured and Compton-thick AGN at high redshifts, revealing their spectral features and challenging some multiwavelength selection methods.

Contribution

First detailed X-ray spectral analysis of heavily obscured AGN at high redshift using ultra-deep XMM-Newton data, highlighting their spectral characteristics and detection challenges.

Findings

Detection of Compton-thick absorption in high-redshift AGN

Identification of reflection-dominated spectra and iron Kalpha lines

Some heavily obscured AGN are missed by multiwavelength selection criteria

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 (CDFS). 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 Kalpha 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 ~ 10^24 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.