The XMM deep survey in the CDF-S

IV. Compton-thick AGN candidates

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: ioannis.georgantopoulos@oabo.inaf.it
² Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
³ Physics & Astronomy Department, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
⁴ Physics Department, University of Durham, South Road, Durham, DH1 7RH, UK
⁵ ICREA and Institut de Ciéncies del COSMOS (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franqués 1, 08028 Barcelona, Spain
⁶ Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), 39005 Santander, Spain
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Irfu/Service Astrophysique, CEA-Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
⁹ INAF Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ Max Planck für Extraterrestrische Physik, Karl Schwarzschild strasse, 85748 Garching, Germany

Received: 30 November 2012
Accepted: 21 March 2013

Abstract

The Chandra Deep Field is the region of the sky with the highest concentration of X-ray data available: 4 Ms of Chandra and 3 Ms of XMM-Newton data, allowing excellent quality spectra to be extracted even for faint sources. We took advantage of this to compile a sample of heavily obscured active galactic nuclei (AGN) using X-ray spectroscopy. We selected our sample among the 176 brightest XMM-Newton sources, searching for either flat X-ray spectra (Γ < 1.4 at the 90% confidence level) suggestive of a reflection dominated continuum or an absorption turn-over suggestive of a column density higher than ≈ 10²⁴ cm^-2. We found a sample of nine heavily-obscured sources satisfying the above criteria. Four of these show statistically significant FeKα lines with large equivalent widths (three out of four have equivalent widths consistent with 1 keV) suggesting that these are the most certain Compton-thick AGN candidates. Two of these sources are transmission dominated while the other two are most probably reflection dominated Compton-thick AGN. Although this sample of four sources is by no means statistically complete, it represents the best example of Compton-thick sources found at moderate-to-high redshift with three sources at z = 1.2–1.5 and one source at z = 3.7. Using Spitzer and Herschel observations, we estimate with good accuracy the X-ray to mid-IR (12  μm) luminosity ratio of our sources. These are well below the average AGN relation, independently suggesting that these four sources are heavily obscured.