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

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Garcet, O.

Aims. Our goal is to characterize AGN populations by comparing their X-ray and optical classifications within the framework of the standard orientationbased unified scheme. Methods. We present a sample of 99 spectroscopically identified (R <= 22 mag) X-ray selected point sources in the XMMLSS survey which are significantly detected (>= 3s) in the [2-10] keV band with fluxes between 8 x 10(-15) and 8 x 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-H(int) >= 10(22) 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 similar to 4 x 10(44) erg s(-1); z similar to 1.9). On the other hand, 25/32 are moderate luminosity ( L2-10 = 5 x 10(43) 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.

Document type Article de périodique (Journal article) – Article de recherche
Access type Accès libre
Publication date 2007
Language Anglais
Journal information "Astronomy & Astrophysics : a European journal" - Vol. 474, no. 2, p. 473-U59 (2007)
Peer reviewed yes
Publisher Edp Sciences S A (Les Ulis Cedex A)
issn 0004-6361
e-issn 1432-0746
Publication status Publié
Affiliation UCL
Keywords X-ray : galaxies ; galaxies : active ; surveys ; galaxies : quasars : emission lines
Links
  1. Akylas A., Georgantopoulos I., Georgakakis A., Kitsionas S., Hatziminaoglou E., XMM-Newton and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift, 10.1051/0004-6361:20054632
  2. Alexander D. M., Bauer F. E., Brandt W. N., Schneider D. P., Hornschemeier A. E., Vignali C., Barger A. J., Broos P. S., Cowie L. L., Garmire G. P., Townsley L. K., Bautz M. W., Chartas G., Sargent W. L. W., The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs, 10.1086/376473
  3. Antonucci Robert, Unified Models for Active Galactic Nuclei and Quasars, 10.1146/annurev.aa.31.090193.002353
  4. Aretxaga Itziar, Terlevich Roberto J., Boyle B. J., Multicolour imaging of z = 2 QSO hosts, 10.1046/j.1365-8711.1998.01398.x
  5. Barcons X., Carrera F. J., Ceballos M. T., H1320+551: a type 1.8/1.9 Seyfert galaxy with an unabsorbed X-ray spectrum, 10.1046/j.1365-8711.2003.06238.x
  6. Bassani, ApJ, 121, 473 (1999)
  7. Bondi M., Ciliegi P., Zamorani G., Gregorini L., Vettolani G., Parma P., de Ruiter H., Le Fevre O., Arnaboldi M., Guzzo L., Maccagni D., Scaramella R., Adami C., Bardelli S., Bolzonella M., Bottini D., Cappi A., Foucaud S., Franzetti P., Garilli B., Gwyn S., Ilbert O., Iovino A., Le Brun V., Marano B., Marinoni C., McCracken H. J., Meneux B., Pollo A., Pozzetti L., Radovich M., Ripepi V., Rizzo D., Scodeggio M., Tresse L., Zanichelli A., Zucca E., The VLA-VIRMOS Deep Field : I. Radio observations probing the μJy source population, 10.1051/0004-6361:20030382
  8. Boyle B. J., Shanks T., Croom S. M., Smith R. J., Miller L., Loaring N., Heymans C., The 2dF QSO Redshift Survey -- I. The optical luminosity function of quasi-stellar objects, 10.1046/j.1365-8711.2000.03730.x
  9. Brandt W. N., Laor A., Wills Beverley J., On the Nature of Soft X‐Ray Weak Quasi‐stellar Objects, 10.1086/308207
  10. Burkert, A&A, 278, 23 (1993)
  11. Caccianiga A., Severgnini P., Braito V., Della Ceca R., Maccacaro T., Wolter A., Barcons X., Carrera F. J., Lehmann I., Page M. J., Saxton R., Webb N. A., The XMM-Newton HBS28 sample: Studying the obscuration in hard X-ray selected AGNs , 10.1051/0004-6361:20034148
  12. Carrera F. J., Ebrero J., Mateos S., Ceballos M. T., Corral A., Barcons X., Page M. J., Rosen S. R., Watson M. G., Tedds J. A., Della Ceca R., Maccacaro T., Brunner H., Freyberg M., Lamer G., Bauer F. E., Ueda Y., The XMM-Newton serendipitous survey : III. The AXIS X-ray source counts and angular clustering, 10.1051/0004-6361:20066271
  13. Cash W., Parameter estimation in astronomy through application of the likelihood ratio, 10.1086/156922
  14. Chiappetti L., Tajer M., Trinchieri G., Maccagni D., Maraschi L., Paioro L., Pierre M., Surdej J., Garcet O., Gosset E., Le Fèvre O., Bertin E., McCracken H. J., Mellier Y., Foucaud S., Radovich M., Ripepi V., Arnaboldi M., The XMM-LSS survey : The XMDS/VVDS 4σ catalogue , 10.1051/0004-6361:20042583
  15. Cohen A. S., Rottgering H. J. A., Kassim N. E., Cotton W. D., Perley R. A., Wilman R., Best P., Pierre M., Birkinshaw M., Bremer M., Zanichelli A., The Low‐Frequency Radio Counterpart of theXMMLarge‐Scale Structure Survey, 10.1086/375397
  16. Coleman G. D., Wu C.-C., Weedman D. W., Colors and magnitudes predicted for high redshift galaxies, 10.1086/190674
  17. Comastri, A&A, 296, 1 (1995)
  18. Comastri A., Fiore F., Vignali C., Matt G., Perola G. C., La Franca F., The BeppoSAX High Energy Large Area Survey (HELLAS) -- III. Testing synthesis models for the X-ray background, 10.1046/j.1365-8711.2001.04737.x
  19. AN, 324, 28 (2003)
  20. Della Ceca R., Maccacaro T., Caccianiga A., Severgnini P., Braito V., Barcons X., Carrera F. J., Watson M. G., Tedds J. A., Brunner H., Lehmann I., Page M. J., Lamer G., Schwope A., Exploring the X-ray sky with the XMM-Newton bright serendipitous survey , 10.1051/0004-6361:20040252
  21. Dickey John M., Lockman Felix J., H I in the Galaxy, 10.1146/annurev.aa.28.090190.001243
  22. Downes A. J. B., Peacock J. A., Savage A., Carrie D. R., The Parkes selected regions: powerful radio galaxies and quasars at high redshifts, 10.1093/mnras/218.1.31
  23. Dye S., Warren S. J., Hambly N. C., Cross N. J. G., Hodgkin S. T., Irwin M. J., Lawrence A., Adamson A. J., Almaini O., Edge A. C., Hirst P., Jameson R. F., Lucas P. W., Van Breukelen C., Bryant J., Casali M., Collins R. S., Dalton G. B., Davies J. I., Davis C. J., Emerson J. P., Evans D. W., Foucaud S., Gonzales-Solares E. A., Hewett P. C., Kendall T. R., Kerr T. H., Leggett S. K., Lodieu N., Loveday J., Lewis J. R., Mann R. G., McMahon R. G., Mortlock D. J., Nakajima Y., Pinfield D. J., Rawlings M. G., Read M. A., Riello M., Sekiguchi K., Smith A. J., Sutorius E. T. W., Varricatt W., Walton N. A., Weatherley S. J., The UKIRT Infrared Deep Sky Survey Early Data Release, 10.1111/j.1365-2966.2006.10928.x
  24. Eckart Megan E., Stern Daniel, Helfand David J., Harrison Fiona A., Mao Peter H., Yost Sarah A., The Serendipitous Extragalactic X‐Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy, 10.1086/504524
  25. Elitzur, M. 2006, ApJS, The Central Engine of Active Galactic Nuclei, ed. L. C. Ho, & J.-M. Wang [arXiv:astro-ph/0612458], in press
  26. Elvis M., Schreier E. J., Tonry J., Davis M., Huchra J. P., Two optically dull galaxies with strong nuclear X-ray sources, 10.1086/158894
  27. Fiore F., La Franca F., Giommi P., Elvis M., Matt G., Comastri A., Molendi S., Gioia I., The contribution of faint active galactic nuclei to the hard X-ray background, 10.1046/j.1365-8711.1999.02795.x
  28. Fiore F., Brusa M., Cocchia F., Baldi A., Carangelo N., Ciliegi P., Comastri A., La Franca F., Maiolino R., Matt G., Molendi S., Mignoli M., Perola G. C., Severgnini P., Vignali C., The HELLAS2XMM survey : IV. Optical identifications and the evolution of the accretion luminosity in the Universe , 10.1051/0004-6361:20031031
  29. Gallagher S. C., Brandt W. N., Chartas G., Priddey R., Garmire G. P., Sambruna R. M., An ExploratoryChandraSurvey of a Well‐defined Sample of 35 Large Bright Quasar Survey Broad Absorption Line Quasars, 10.1086/503762
  30. Gandhi P., Fabian A. C., X-ray background synthesis: the infrared connection, 10.1046/j.1365-8711.2003.06259.x
  31. Gandhi P., Crawford C. S., Fabian A. C., Johnstone R. M., Powerful, obscured active galactic nuclei among X-ray hard, optically dim serendipitousChandrasources, 10.1111/j.1365-2966.2004.07341.x
  32. Gandhi P., Garcet O., Disseau L., Pacaud F., Pierre M., Gueguen A., Alloin D., Chiappetti L., Gosset E., Maccagni D., Surdej J., Valtchanov I., The XMM large scale structure survey: properties and two-point angular correlations of point-like sources, 10.1051/0004-6361:20065284
  33. Gavignaud I., Bongiorno A., Paltani S., Mathez G., Zamorani G., Møller P., Picat J. P., Le Brun V., Marano B., Le Fèvre O., Bottini D., Garilli B., Maccagni D., Scaramella R., Scodeggio M., Tresse L., Vettolani G., Zanichelli A., Adami C., Arnaboldi M., Arnouts S., Bardelli S., Bolzonella M., Cappi A., Charlot S., Ciliegi P., Contini T., Foucaud S., Franzetti P., Guzzo L., Ilbert O., Iovino A., McCracken H. J., Marinoni C., Mazure A., Meneux B., Merighi R., Pellò R., Pollo A., Pozzetti L., Radovich M., Zucca E., Bondi M., Busarello G., Cucciati O., de la Torre S., Gregorini L., Lamareille F., Mellier Y., Merluzzi P., Ripepi V., Rizzo D., Vergani D., The VIMOS VLT Deep Survey: the faint type-1 AGN sample , 10.1051/0004-6361:20065376
  34. Gehrels N., Confidence limits for small numbers of events in astrophysical data, 10.1086/164079
  35. Giacconi Riccardo, Zirm Andrew, Wang JunXian, Rosati Piero, Nonino Mario, Tozzi Paolo, Gilli Roberto, Mainieri Vincenzo, Hasinger Guenther, Kewley Lisa, Bergeron Jacqueline, Borgani Stefano, Gilmozzi Roberto, Grogin Norman, Koekemoer Anton, Schreier Ethan, Zheng Wei, Norman Colin, ChandraDeep Field South: The 1 Ms Catalog, 10.1086/338927
  36. Gilli R., Comastri A., Hasinger G., The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era, 10.1051/0004-6361:20066334
  37. Griffiths, MNRAS, 275, 77 (1995)
  38. Hickox Ryan C., Markevitch Maxim, Absolute Measurement of the Unresolved Cosmic X‐Ray Background in the 0.5–8 keV Band withChandra, 10.1086/504070
  39. Kewley L. J., Groves B., Kauffmann G., Heckman T., The host galaxies and classification of active galactic nuclei, 10.1111/j.1365-2966.2006.10859.x
  40. Kukula Marek J., Dunlop James S., McLure Ross J., Miller Lance, Percival Will J., Baum Stefi A., O'Dea Christopher P., A NICMOS imaging study of high-zquasar host galaxies, 10.1111/j.1365-2966.2001.04692.x
  41. Lamareille F., Mouhcine M., Contini T., Lewis I., Maddox S., The luminosity-metallicity relation in the local Universe from the 2dF Galaxy Redshift Survey, 10.1111/j.1365-2966.2004.07697.x
  42. Lawrence A., Warren S. J., Almaini O., Edge A. C., Hambly N. C., Jameson R. F., Lucas P., Casali M., Adamson A., Dye S., Emerson J. P., Foucaud S., Hewett P., Hirst P., Hodgkin S. T., Irwin M. J., Lodieu N., McMahon R. G., Simpson C., Smail I., Mortlock D., Folger M., The UKIRT Infrared Deep Sky Survey (UKIDSS), 10.1111/j.1365-2966.2007.12040.x
  43. Le Fèvre O., Mellier Y., McCracken H. J., Foucaud S., Gwyn S., Radovich M., Dantel-Fort M., Bertin E., Moreau C., Cuillandre J.-C., Pierre M., Le Brun V., Mazure A., Tresse L., The VIRMOS deep imaging survey : I. Overview, survey strategy, and CFH12K observations, 10.1051/0004-6361:20031767
  44. Le Fèvre O., Vettolani G., Garilli B., Tresse L., Bottini D., Le Brun V., Maccagni D., Picat J. P., Scaramella R., Scodeggio M., Zanichelli A., Adami C., Arnaboldi M., Arnouts S., Bardelli S., Bolzonella M., Cappi A., Charlot S., Ciliegi P., Contini T., Foucaud S., Franzetti P., Gavignaud I., Guzzo L., Ilbert O., Iovino A., McCracken H. J., Marano B., Marinoni C., Mathez G., Mazure A., Meneux B., Merighi R., Paltani S., Pellò R., Pollo A., Pozzetti L., Radovich M., Zamorani G., Zucca E., Bondi M., Bongiorno A., Busarello G., Lamareille F., Mellier Y., Merluzzi P., Ripepi V., Rizzo D., The VIMOS VLT deep survey : First epoch VVDS-deep survey: 11 564 spectra with 17.5 ≤ I$_\textit{\textbf{\small AB}}$ ≤ 24, and the redshift distribution over 0 ≤ z ≤ 5 , 10.1051/0004-6361:20041960
  45. Lonsdale Carol J., Smith Harding E., Rowan‐Robinson Michael, Surace Jason, Shupe David, Xu Cong, Oliver Sebastian, Padgett Deborah, Fang Fan, Conrow Tim, Franceschini Alberto, Gautier Nick, Griffin Matt, Hacking Perry, Masci Frank, Morrison Glenn, O’Linger Joanne, Owen Frazer, Pérez‐Fournon Ismael, Pierre Marguerite, Puetter Rick, Stacey Gordon, Castro Sandra, Del Carmen Polletta Maria, Farrah Duncan, Jarrett Tom, Frayer Dave, Siana Brian, Babbedge Tom, Dye Simon, Fox Matt, Gonzalez‐Solares Eduardo, Salaman Malcolm, Berta Stefano, Condon Jim J., Dole Hervé, Serjeant Steve, SWIRE: TheSIRTFWide‐Area Infrared Extragalactic Survey, 10.1086/376850
  46. Mainieri V., Bergeron J., Hasinger G., Lehmann I., Rosati P., Schmidt M., Szokoly G., Della Ceca R., XMM-Newton observation of the Lockman Hole : II. Spectral analysis, 10.1051/0004-6361:20020991
  47. Mainieri, V., Hasinger, G., Cappelluti, N., et al. 2006, ApJS COSMOS [arXiv:astro-ph/0612361], accepted
  48. Maiolino R., Marconi A., Salvati M., Risaliti G., Severgnini P., Oliva E., La Franca F., Vanzi L., Dust in active nuclei : I. Evidence for "anomalous"properties, 10.1051/0004-6361:20000177
  49. Maiolino R., Marconi A., Oliva E., Dust in active nuclei : II. Powder or gravel?, 10.1051/0004-6361:20000012
  50. Mateos S., Barcons X., Carrera F. J., Ceballos M. T., Caccianiga A., Lamer G., Maccacaro T., Page M. J., Schwope A., Watson M. G., X-ray spectra of XMM-Newton serendipitous medium flux sources, 10.1051/0004-6361:20041340
  51. Moran Edward C., Filippenko Alexei V., Chornock Ryan, “Hidden” Seyfert 2 Galaxies and the X-Ray Background, 10.1086/345314
  52. Murray N., Chiang J., Grossman S. A., Voit G. M., Accretion Disk Winds from Active Galactic Nuclei, 10.1086/176238
  53. Nandra K., Pounds K. A., Ginga observations of the X-ray spectra of Seyfert galaxies, 10.1093/mnras/268.2.405
  54. Pacaud F., Pierre M., Refregier A., Gueguen A., Starck J.-L., Valtchanov I., Read A. M., Altieri B., Chiappetti L., Gandhi P., Garcet O., Gosset E., Ponman T. J., Surdej J., TheXMMLarge-Scale Structure survey: the X-ray pipeline and survey selection function, 10.1111/j.1365-2966.2006.10881.x
  55. Page M. J., Mason K. O., McHardy I. M., Jones L. R., Carrera F. J., The evolution of QSOs derived from soft X-ray surveys, 10.1093/mnras/291.2.324
  56. Page M. J., Mittaz J. P. D., Carrera F. J., A survey of hard spectrum ROSAT sources - II. Optical identification of hard sources, 10.1046/j.1365-8711.2001.04436.x
  57. Page, AN, 324, 101 (2003)
  58. Page M. J., Stevens J. A., Ivison R. J., Carrera F. J., The Evolutionary Sequence of Active Galactic Nuclei and Galaxy Formation Revealed, 10.1086/423892
  59. Page M. J., Loaring N. S., Dwelly T., Mason K. O., McHardy I., Gunn K., Moss D., Sasseen T., Cordova F., Kennea J., Seymour N., X-ray spectra of sources in the 13HXMM–Newton/Chandra deep field, 10.1111/j.1365-2966.2006.10278.x
  60. Page, M. J., Carrera, F. J., Ebrero, J., et al. 2006b, in conference proceedings, Studying Galaxy Evolution with Spitzer and Herschel, in press
  61. Panessa F., Bassani L., Unabsorbed Seyfert 2 galaxies, 10.1051/0004-6361:20021161
  62. Pappa A., Georgantopoulos I., Stewart G. C., Zezas A. L., The X-ray spectra of optically selected Seyfert 2 galaxies: are there any Seyfert 2 galaxies with no absorption?, 10.1046/j.1365-8711.2001.04609.x
  63. Perola G. C., Puccetti S., Fiore F., Sacchi N., Brusa M., Cocchia F., Baldi A., Carangelo N., Ciliegi P., Comastri A., La Franca F., Maiolino R., Matt G., Mignoli M., Molendi S., Vignali C., The HELLAS2XMM survey : VI. X-ray absorption in the 1df AGN sample through a spectral analysis, 10.1051/0004-6361:20047118
  64. Pierre Marguerite, Valtchanov Ivan, Altieri Bruno, Andreon Stefano, Bolzonella Micol, Bremer Malcolm, Disseau Ludovic, Santos Sergio Dos, Gandhi Poshak, Jean Christophe, Pacaud Florian, Read Andrew, Refregier Alexandre, Willis Jon, Adami Christophe, Alloin Danielle, Birkinshaw Mark, Chiappetti Lucio, Cohen Aaron, Detal Alain, Duc Pierre-Alain, Gosset Eric, Hjorth Jens, Jones Laurence, Fevre Olivier Le, Lonsdale Carol, Maccagni Dario, Mazure Alain, McBreen Brian, McCracken Henry, Mellier Yannick, Ponman Trevor, Quintana Hernan, Rottgering Huub, Smette Alain, Surdej Jean, Starck Jean-Luc, Vigroux Laurent, White Simon, The XMM-LSS survey. Survey design and first results, 10.1088/1475-7516/2004/09/011
  65. Pierre M., Pacaud F., Duc P.-A., Willis J. P., Andreon S., Valtchanov I., Altieri B., Galaz G., Gueguen A., Fèvre J.-P. Le, Fèvre O. Le, Ponman T., Sprimont P.-G., Surdej J., Adami C., Alshino A., Bremer M., Chiappetti L., Detal A., Garcet O., Gosset E., Jean C., Maccagni D., Marinoni C., Mazure A., Quintana H., Read A., TheXMMLarge-Scale Structure survey: a well-controlled X-ray cluster sample over the D1 CFHTLS area, 10.1111/j.1365-2966.2006.10886.x
  66. Pierre, M., Chiappetti, L., Pacaud, F., et al. 2007, MNRAS [arXiv:astro-ph/07083299v1]
  67. Polletta M., Tajer M., Maraschi L., Trinchieri G., Lonsdale C. J., Chiappetti L., Andreon S., Pierre M., Le Fevre O., Zamorani G., Maccagni D., Garcet O., Surdej J., Franceschini A., Alloin D., Shupe D. L., Surace J. A., Fang F., Rowan‐Robinson M., Smith H. E., Tresse L., Spectral Energy Distributions of Hard X‐Ray Selected Active Galactic Nuclei in theXMM‐NewtonMedium Deep Survey, 10.1086/518113
  68. Punsly Brian, X‐Ray Absorption in Type II Quasars: Implications for the Equatorial Paradigm of Broad Absorption Line Quasars, 10.1086/505482
  69. Ranalli P., Comastri A., Setti G., The 2–10 keV luminosity as a Star Formation Rate indicator , 10.1051/0004-6361:20021600
  70. Risaliti G., Marconi A., Maiolino R., Salvati M., Severgnini P., A new population of soft X-ray weak quasars, 10.1051/0004-6361:20010276
  71. Setti, A&A, 224, L21 (1989)
  72. Severgnini P., Caccianiga A., Braito V., Della Ceca R., Maccacaro T., Wolter A., Sekiguchi K., Sasaki T., Yoshida M., Akiyama M., Watson M. G., Barcons X., Carrera F. J., Pietsch W., Webb N. A., XMM-Newton observations reveal AGN in apparently normal galaxies, 10.1051/0004-6361:20030625
  73. Severgnini P., Della Ceca R., Braito V., Saracco P., Longhetti M., Bender R., Drory N., Feulner G., Hopp U., Mannucci F., Maraston C., Looking for obscured QSOs in the X-ray emitting ERO population, 10.1051/0004-6361:20041587
  74. Silverman J. D., Green P. J., Barkhouse W. A., Kim D.‐W., Aldcroft T. L., Cameron R. A., Wilkes B. J., Mossman A., Ghosh H., Tananbaum H., Smith M. G., Smith R. C., Smith P. S., Foltz C., Wik D., Jannuzi B. T., Hard X‐Ray–emitting Active Galactic Nuclei Selected by theChandraMultiwavelength Project, 10.1086/425895
  75. Spergel, ApJ, 170, 377 (2007)
  76. Tajer M., Polletta M., Chiappetti L., Maraschi L., Trinchieri G., Maccagni D., Andreon S., Garcet O., Surdej J., Pierre M., Le Fèvre O., Franceschini A., Lonsdale C. J., Surace J. A., Shupe D. L., Fang F., Rowan-Robinson M., Smith H. E., Tresse L., Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey, 10.1051/0004-6361:20066667
  77. Treister Ezequiel, Urry C. Megan, Active Galactic Nuclei Unification and the X‐Ray Background, 10.1086/431892
  78. Treister Ezequiel, Castander Francisco J., Maccarone Thomas J., Gawiser Eric, Coppi Paolo S., Urry C. Megan, Maza Jose, Herrera David, Gonzalez Valentino, Montoya Carlos, Pineda Pedro, The Calan‐Yale Deep Extragalactic Research (CYDER) Survey: Optical Properties and Deep Spectroscopy of Serendipitous X‐Ray Sources, 10.1086/427471
  79. Turner T. J., Pounds K. A., The EXOSAT spectral survey of AGN, 10.1093/mnras/240.4.833
  80. Veilleux Sylvain, Osterbrock Donald E., Spectral classification of emission-line galaxies, 10.1086/191166
  81. Weingartner Joseph C., Murray Norman, X‐Ray versus Optical Observations of Active Galactic Nuclei: Evidence for Large Grains?, 10.1086/343115
  82. Worsley M. A., Resolving the X-ray Background, 10.1063/1.2141832
Bibliographic reference Garcet, O. ; et. al. The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme. In: Astronomy & Astrophysics : a European journal, Vol. 474, no. 2, p. 473-U59 (2007)
Permanent URL http://hdl.handle.net/2078.1/37297