A multi-wavelength survey of AGN in the XMM-LSS field

I. Quasar selection via the KX technique

¹ Sterrenkundig Observatorium, University of Ghent, Krijgslaan 281 S9, 9000 Ghent, Belgium e-mail: theodoros.nakos@ugent.be
² Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 août, 17, B5c, 4000 Sart Tilman, Belgium
³ Department of Physics and Astronomy, Univ. of Victoria, Elliott Building 3, 800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
⁴ Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁵ 60 rue des Bergers, 75015 Paris, France
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁷ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât. 709, CEA Saclay, 91191 Gif sur Yvette Cedex, France
⁸ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
¹⁰ Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK
¹¹ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
¹² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 15 February 2008
Accepted: 6 December 2008

Abstract

Aims. We present a sample of candidate quasars selected using the KX-technique. The data cover 0.68 deg² of the X-ray Multi-Mirror (XMM) Large-Scale Structure (LSS) survey area where overlapping multi-wavelength imaging data permits an investigation of the physical nature of selected sources.

Methods. The KX method identifies quasars on the basis of their optical (R and z') to near-infrared photometry and point-like morphology. We combine these data with optical (u^*,g', r',i',z') and mid-infrared (3.6-24  μm) wavebands to reconstruct the spectral energy distributions (SEDs) of candidate quasars.

Results. Of 93 sources selected as candidate quasars by the KX method, 25 are classified as quasars by the subsequent SED analysis. Spectroscopic observations are available for 12/25 of these sources and confirm the quasar hypothesis in each case. Even more, 90% of the SED-classified quasars show X-ray emission, a property not shared by any of the false candidates in the KX-selected sample. Applying a photometric redshift analysis to the sources without spectroscopy indicates that the 25 sources classified as quasars occupy the interval 0.7 ≤ z ≤ 2.5. The remaining 68/93 sources are classified as stars and unresolved galaxies.