Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey^*

¹ INAF – Osservatorio di Brera, via Brera 28, 20121 Milano, Italy e-mail: marzia.tajer@brera.inaf.it
² Università degli Studi di Milano – Bicocca, Dipartimento di Fisica, Piazza della Scienza 3, 20126 Milano, Italy
³ Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
⁴ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy
⁵ Institut d'Astrophysique et de Géophysique, Université de Liège, 17 Allée du 6 Août, 4000 Liège 1, Belgium
⁶ CEA/DSM/DAPNIA Service d'Astrophysique, Saclay, 91191 Gif-sur-Yvette, France
⁷ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS-Université de Provence, BP 8, 13376 Marseille Cedex 12, France
⁸ Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy
⁹ Science Center, California Institute of Technology, 100-22 Pasadena, CA 911125, USA
¹⁰ Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London, SW7 2BW, UK

Received: 31 October 2006
Accepted: 2 March 2007

Abstract

Aims.Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample with wide multiwavelength coverage.

Methods.We selected a sample of 136 X-ray sources detected at a significance of ≥ in the  keV band (F_2-10 10^-14 erg cm^-2 s^-1) in a ~1 deg² area in the XMM Medium Deep Survey (XMDS). The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data from the SWIRE survey. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical – IR identification. The observed optical and IR spectral energy distributions of all identified sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. X-ray spectral analysis is performed individually for sources with a sufficient number of counts and using a stacking technique for subsamples of sources at different flux levels. Hardness ratios are used to estimate X-ray absorption in individual weak sources.

Results.70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template. We group them together in a single class of “optically obscured” AGN. These have “red” optical colors and in about 60% of cases show significant X-ray absorption (N_H > 10²² cm^-2). Sources with SEDs typical of type 1 AGN have “blue” optical colors and exhibit X-ray absorption in about 30% of cases. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Γ = 1.6. The subsample of objects fitted by a star forming galaxy template has an even harder stacked spectrum, with 1.2-1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than “optical” obscuration and its incidence is nearly constant with redshift and luminosity. This implies that at high luminosities X-ray absorption is not necessarily related to optical obscuration. The estimated surface densities of obscured, unobscured AGN and type 2 QSOs are respectively 138, 59 and 35 deg^-2 at  erg cm^-2 s^-1.