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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
earth and planetary sciences
The XMM large scale structure survey: Optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Astronomy and Astrophysics, Volume 474, No. 2, Year 2007
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Description
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 (NHint ≥ 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 (L2-10 ∼ 4 × 10 44 erg s-1; z ∼ 1.9). On the other hand, 25/32 are moderate luminosity (L2-10 ≤ 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. © ESO 2007.
Authors & Co-Authors
Garcet, O.
Unknown Affiliation
Gandhi, Poshak R.
Unknown Affiliation
Gösset, Eric
Unknown Affiliation
Sprimont, Pierre G.
Unknown Affiliation
Surdej, Jean
Unknown Affiliation
Borkowski, V.
Unknown Affiliation
Tajer, M.
Unknown Affiliation
Pacaud, Florian
Unknown Affiliation
Pierre, Marguerite M.
Unknown Affiliation
Chiappetti, Lucio
Unknown Affiliation
MacCagni, Dario
Unknown Affiliation
Page, Mathew J.
Unknown Affiliation
Carrera, Francisco J.
Unknown Affiliation
Tedds, Johnathan A.
Unknown Affiliation
Mateos, Silvia
Unknown Affiliation
Krumpe, Mirko
Unknown Affiliation
Contini, T.
Unknown Affiliation
Corral, A.
Unknown Affiliation
Ebrero, Jacobo
Unknown Affiliation
Gavignaud, I.
Unknown Affiliation
Schwope, Axel D.
Unknown Affiliation
Le Fèvre, Olivier
Unknown Affiliation
Polletta, Maria Del Carmen
Unknown Affiliation
Rosen, Simon R.
Unknown Affiliation
Lonsdale, Carol J.
Unknown Affiliation
Watson, Mike G.
Unknown Affiliation
Borczyk, W.
Unknown Affiliation
Väisänen, Petri S.M.
Unknown Affiliation
Statistics
Citations: 28
Authors: 28
Affiliations: 16
Identifiers
Doi:
10.1051/0004-6361:20077778
ISSN:
00046361
e-ISSN:
14320746
Research Areas
Environmental
Study Design
Cross Sectional Study
Study Approach
Quantitative