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The XMM-Newton Wide-Field Survey in the COSMOS Field. III. Optical Identification and Multiwavelength Properties of a Large Sample of X-Ray-Selected Sources

Brusa, M. and Zamorani, G. and Comastri, A. and Hasinger, G. and Cappelluti, N. and Civano, F. and Finoguenov, A. and Mainieri, V. and Salvato, M. and Vignali, C. and Elvis, M. and Fiore, F. and Gilli, R. and Impey, C. D. and Lilly, S. J. and Mignoli, M. and Silverman , J. and Trump, J. and Urry, C. M. and Bender, R. and Capak, P. and Huchra, J. P. and Kneib, J. P. and Koekemoer, A. and Leauthaud, A. and Lehmann, I. and Massey, R. and Matute, I. and McCarthy, P. J. and McCracken, H. J. and Rhodes, J. and Scoville, N. Z. and Taniguchi, Y. and Thompson, D. (2007) The XMM-Newton Wide-Field Survey in the COSMOS Field. III. Optical Identification and Multiwavelength Properties of a Large Sample of X-Ray-Selected Sources. Astrophysical Journal Supplement Series, 172 (1). pp. 353-367. ISSN 0067-0049.

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We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg^2 of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^(-15) erg cm^(-2) s^(-1) (~5 × 10^(-15) erg cm^(-2) s^(-1)). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray-to-optical associations we have used the "likelihood ratio technique." Here we present the identification method and its application to the CFHT I-band and photometric catalogs. We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I_(AB) > 25) or to the presence of multiple optical sources, with similar likelihoods of being the correct identification, within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS, and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis of this statistically meaningful sample of X-ray sources reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical-to-near-infrared colors: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray-to-optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical-to-near-infrared colors, and span a larger range of X-ray-to-optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the "extended" BL AGNs lie at redshift <1.5, while the redshift distribution of the full BL AGN population peaks at z ~ 1.5. The most likely explanation is that in these objects the nuclear emission is not dominant with respect to the host galaxy emission in the observed ACS band. Our analysis also suggests that the type 2/type 1 ratio decreases toward high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.

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Additional Information:© 2007 American Astronomical Society. Received 2006 April 24; accepted 2006 November 21. 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). In Germany, the XMM-Newton project is supported by the Bundesministerium fur Wirtschaft und Technologie/Deutsches Zentrum fur Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OX 0001), the Max-Planck Society, and the Heidenhain-Stiftung. Part of this work was supported by the Deutsches Zentrum fur Luft und Raumfahrt, DLR project numbers 50 OR 0207 and 50OR 0405. In Italy, the COSMOS XMM-Newton project is supported by ASI-INAF and MIUR under grants I/023/05/00 and Cofin-03-02-23. This work is based in part on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), and on data products produced at TERAPIX data center, located at the Institute d’Astrophysique de Paris. M. B. and G. Z. gratefully acknowledge useful discussion with G. Micela and S. Sciortino. We thank the anonymous referee for her /his useful comments on this manuscript. We gratefully acknowledge the contributions of the entire COSMOS collaboration; more information on the COSMOS survey is available at This research has made use of the NASA/IPAC Extragalactic Database (NED) and the SDSS spectral archive.
Subject Keywords:galaxies: active; surveys; X-rays: diffuse background; X-rays: galaxies; X-rays: general
Record Number:CaltechAUTHORS:20100421-095358500
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Official Citation:M. Brusa et al 2007 ApJS 172 353 doi: 10.1086/516575
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18037
Deposited By: Jason Perez
Deposited On:21 Apr 2010 18:13
Last Modified:06 Jul 2015 21:22

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