The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme^*

¹ Institut d'Astrophysique et de Géophysique, Université de Liège, Belgium
² RIKEN Cosmic Radiation Lab, 2-1 Hirosawa, Wakoshi, Saitama 351-0198, Japan
³ INAF – Osservatorio di Brera, via Brera 28, 20121 Milano, Italy
⁴ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza della Scienza 3, 20126 Milano, Italy
⁵ CEA/DSM/DAPNIA Service d'astrophysique, Saclay, 91191 Gif-sur-Yvette, France
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ INAF-IASF Milano, via Bassini 15, 20133 Milano, Italy
⁸ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
⁹ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
¹⁰ Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK
¹¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹² Laboratoire d'Astrophysique de Toulouse et Tarbes (UMR 5572), Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
¹³ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS-Université de Provence, BP 8, 13376 Marseille Cedex 12, France
¹⁴ Center for Astrophysics & Space Sciences, University of California, San Diego, La Jolla, CA 92093-0424, USA
¹⁵ Astronomical Observatory of A. Mickiewicz University, Sloneczna 36, Poznań 60-286, Poland
¹⁶ South African Astronomical Observatory, Observatory 7935, South Africa

Received: 2 May 2007
Accepted: 28 August 2007

Abstract

Aims.Our goal is to characterize AGN populations by comparing their X-ray and optical classifications within the framework of the standard orientation-based unified scheme.

Methods.We present a sample of 99 spectroscopically identified (R ≤ 22 mag) X-ray selected point sources in the XMM-LSS survey which are significantly detected (≥3σ) in the [ 2-10]  keV band with fluxes between 8  10^-15 and 8  10^-14 erg s^-1 cm^-2, and which have more than 80 counts. We have compared their X-ray and optical classifications. To this end, we performed an X-ray spectral analysis for all of these 99 X-ray sources in order to assess whether they are intrinsically absorbed or not. The X-ray classification is based on the measured intrinsic column density. The optical classification is based on the measured FWHM of the permitted emission lines, the absence of broad lines being due to obscuration within the framework of the standard AGN unified scheme.

Results.Introducing the fourfold point correlation coefficient r, we find a mild correlation between the X-ray and the optical classifications (r = 0.28), as up to 32 X-ray sources out of 99 have differing X-ray and optical classifications: on one hand, 10% of the type 1 sources (7/32) present broad emission lines in their optical spectra and strong absorption (N ≥ 10²² cm^-2) in the X-rays. These objects are highly luminous AGN lying at high redshift and thus dilution effects by the host galaxy light are totally ruled out, their discrepant nature being an intrinsic property instead. Their X-ray luminosities and redshifts distributions are consistent with those of the unabsorbed X-ray sources with broad emission lines (L_2-10 ~ 4  10⁴⁴ erg s^-1; z ~ 1.9). On the other hand, 25/32 are moderate luminosity (L_2-10 ≤ 5  10⁴³ erg s^-1) AGN, which are both unabsorbed in the X-rays and only present narrow emission lines in their optical spectra. Based on their line ratios in the optical, the majority of them have an optical spectrum which is more representative of the host galaxy rather than of a reddened AGN. We finally infer that dilution of the AGN by the host galaxy seems to account for their nature. 5/25 have been defined as Seyfert 2 based on their optical spectra. In conclusion, most of these 32 discrepant cases can be accounted for by the standard AGN unified scheme, as its predictions are not met for only 12% of the 99 X-ray sources.