The XMM deep survey in the CDF-S

II. A 9–20 keV selection of heavily obscured active galaxies at z  >  1.7

¹ ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Università di Bologna – Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
⁴ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
⁷ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
⁸ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁹ Dipartimento di Scienze Fisiche, Università di Napoli Fedelico II, C.U. di Monte Sant’Angelo, via Cintia ed. 6, 80126 Napoli, Italy

Received: 17 July 2012
Accepted: 4 September 2012

Abstract

We present results on a search of heavily obscured active galaxies z > 1.7 using the rest-frame 9−20 keV excess for X-ray sources detected in the deep XMM-CDFS survey. Out of 176 sources selected with the conservative detection criteria (>8σ) in the first source catalogue of Ranalli et al. (in prep.), 46 objects lie in the redshift range of interest with the median redshift $\begin{array}{}{\mathrm{˜}}_{}\\ \mathit{z}\end{array}$ ≃ 2.5. Their typical rest-frame 10−20 keV luminosity is 10⁴⁴ erg s^-1, as observed. Among optically faint objects that lack spectroscopic redshift, four were found to be strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption features in their X-ray spectra were used to obtain X-ray spectroscopic redshifts. Using the X-ray colour–colour diagram based on the rest-frame 3−5 keV, 5−9 keV, and 9−20 keV bands, seven objects were selected for their 9−20 keV excess and were found to be strongly absorbed X-ray sources with column density of N_H ≥ 0.6 × 10²⁴ cm^-2, including two possible Compton thick sources. While they are emitting at quasar luminosity, ~3/4 of the sample objects are found to be absorbed by N_H > 10²² cm^-2. A comparison with local AGN at the matched luminosity suggests an increasing trend of the absorbed source fraction for high-luminosity AGN towards high redshifts.