A Caltech Library Service

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

Tajer, M. and Polletta, M. and Chiappetti, L. and Maraschi, L. and Trincheri, G. and Maccagni, D. and Andreon, S. and Garcet, O. and Surdej, J. and Pierre, M. and Le Fèvre, O. and Franceschini, A. and Lonsdale, C. J. and Surace, J. A. and Shupe, D. L. and Fang, F. and Rowan-Robinson, M. and Smith, H. E. and Tresse, L. (2007) Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey. Astronomy and Astrophysics, 467 (1). pp. 73-91. ISSN 0004-6361.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


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 ≥3σ in the 2-10 keV band (F_(2-10) ≳ 10^(-14) erg cm^(-2) s^(-1)) in a ~1 deg^2 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 > 10^(22) 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 Γ ~ 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 F > 10^(-14) erg cm^(-2) s^(-1).

Item Type:Article
Related URLs:
URLURL TypeDescription
Polletta, M.0000-0001-7411-5386
Surdej, J.0000-0002-7005-1976
Le Fèvre, O.0000-0001-5891-2596
Lonsdale, C. J.0000-0003-0898-406X
Surace, J. A.0000-0001-7291-0087
Shupe, D. L.0000-0003-4401-0430
Additional Information:© 2007 ESO. Received 31 October 2006; accepted 2 March 2007. We thank M. Bolzonella for her suggestions in computing the photometric redshifts and her comments on the paper and A. Bongiorno for helping us with the VVDS spectra. We also thank D. Alloin, R. Della Ceca, P. Severgnini and G. Zamorani for useful comments. M.T. acknowledges financial support from MIUR Cofin 2004-023189-005. The INAF members of the team acknowledge financial contribution from contract ASI-INAF I/023/05/0. O.G. and J.S. would like to acknowledge support by contract Inter-University Attraction Pole P5/36 (Belgium), the ESA PRODEX Programme (XMM-LSS) and the Belgium Federal Science Policy Office. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). Also based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. This work is in part based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR)2004-023189-005
Istituto Nazionale di Astrofisica (INAF)ASI-INAF I/023/05/0
Inter-University Attraction Pole P5/36 (Belgium)UNSPECIFIED
Belgian Federal Science Policy Office (BELSPO)UNSPECIFIED
Subject Keywords:X-rays: general; surveys; galaxies: active
Issue or Number:1
Record Number:CaltechAUTHORS:20100112-113609827
Persistent URL:
Official Citation:Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey M. Tajer, M. Polletta, L. Chiappetti, L. Maraschi, G. Trinchieri, D. Maccagni, S. Andreon, O. Garcet, J. Surdej, M. Pierre, O. Le Fèvre, A. Franceschini, C. J. Lonsdale, J. A. Surace, D. L. Shupe, F. Fang, M. Rowan-Robinson, H. E. Smith and L. Tresse A&A 467 (1) 73-91 (2007) DOI: 10.1051/0004-6361:20066667
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17147
Deposited By: Jason Perez
Deposited On:13 Jan 2010 22:20
Last Modified:09 Mar 2020 13:19

Repository Staff Only: item control page