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The Chandra COSMOS Survey. III. Optical and Infrared Identification of X-Ray Point Sources

Civano, F. and Capak, P. and Scoville, N. Z. (2012) The Chandra COSMOS Survey. III. Optical and Infrared Identification of X-Ray Point Sources. Astrophysical Journal Supplement Series, 201 (2). Art. No. 30. ISSN 0067-0049.

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The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of 1.9 × 10^(–16) erg cm^(–2) s^(–1) in the soft (0.5-2 keV) band, 7.3 × 10^(–16) erg cm^(–2) s^(–1) in the hard (2-10 keV) band, and 5.7 × 10^(–16) erg cm^(–2) s^(–1) in the full (0.5-10 keV) band. In this paper we report the i, K, and 3.6 μm identifications of the 1761 X-ray point sources. We use the likelihood ratio technique to derive the association of optical/infrared counterparts for 97% of the X-ray sources. For most of the remaining 3%, the presence of multiple counterparts or the faintness of the possible counterpart prevented a unique association. For only 10 X-ray sources we were not able to associate a counterpart, mostly due to the presence of a very bright field source close by. Only two sources are truly empty fields. The full catalog, including spectroscopic and photometric redshifts and classification described here in detail, is available online. Making use of the large number of X-ray sources, we update the "classic locus" of active galactic nuclei (AGNs) defined 20 years ago in soft X-ray surveys and define a new locus containing 90% of the AGNs in the survey with full-band luminosity >10^(42) erg s^(–1). We present the linear fit between the total i-band magnitude and the X-ray flux in the soft and hard bands, drawn over two orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we test its known correlation with redshift and luminosity, and a recently introduced anti-correlation with the concentration index (C). We find a strong anti-correlation (though the dispersion is of the order of 0.5 dex) between X/O computed in the hard band and C and that 90% of the obscured AGNs in the sample with morphological information live in galaxies with regular morphology (bulgy and disky/spiral), suggesting that secular processes govern a significant fraction of the black hole growth at X-ray luminosities of 10^(43)-10^(44.5) erg s^(–1). We also investigate the degree of obscuration of the sample using the hardness ratio, and we compare the X-ray color with the near-infrared to optical color.

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Additional Information:© 2012 American Astronomical Society. Received 2011 November 18; accepted 2012 May 21; published 2012 August 2. The authors thank the referee for the useful suggestions which helped to improve the paper. F.C. thanks R. D’Abrusco, A. Goulding, M. Mignoli, and M. Povic for inspiring discussions and useful comments. This work was supported in part by NASA Chandra grant number GO7-8136A (M.E., F.C., H.H.), the Blancheflor Boncompagni Ludovisi foundation (F.C.), and the Smithsonian Scholarly Studies (F.C.). In Italy this work is supported by ASI/INAF contracts I/009/10/0 and I/088/06 and by PRIN INAF 2010 “From the dawn of galaxy formation to the peak of themass assembly.” M.S. acknowledges support by the German Deutsche Forschungsgemeinschaft, DFG Leibniz Prize (FKZ HA 1850/28-1). InMexico, this work has been supported by CONACyT Grant 83564 and UNAM/DGAPA PAPIIT Grant IN110209. This work is also based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under program ID 175.A-0839.
Funding AgencyGrant Number
NASA Chandra GrantGO7-8136A
Blancheflor Boncompagni Ludovisi FoundationUNSPECIFIED
Smithsonian Scholarly StudiesUNSPECIFIED
DFG Leibniz PrizeFKZ HA 1850/28-1
CONACyT Grant83564
Subject Keywords:galaxies: active; surveys; X-rays: galaxies: clusters
Record Number:CaltechAUTHORS:20121101-142013998
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Official Citation:The Chandra COSMOS Survey. III. Optical and Infrared Identification of X-Ray Point Sources F. Civano et al. 2012 ApJS 201 30
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35246
Deposited By: Jason Perez
Deposited On:01 Nov 2012 23:06
Last Modified:20 Jul 2015 18:04

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