Volume 546, October 2012
|Number of page(s)||8|
|Published online||09 October 2012|
The XMM deep survey in the CDF-S
II. A 9–20 keV selection of heavily obscured active galaxies at z > 1.7
ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona
Martí i Franquès 1,
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
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 ≃ 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 NH ≥ 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 NH > 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.
Key words: galaxies: active / X-rays: galaxies
© ESO, 2012
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