1205.5030 (F. Civano et al.)

The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources    [PDF]

F. Civano, M. Elvis, M. Brusa, A. Comastri, M. Salvato, G. Zamorani, T. Aldcroft, A. Bongiorno, P. Capak, N. Cappelluti, M. Cisternas, F. Fiore, A. Fruscione, H. Hao, J. Kartaltepe, A. Koekemoer, R. Gilli, C. D. Impey, G. Lanzuisi, E. Lusso, V. Mainieri, T. Miyaji, S. Lilly, D. Masters, S. Puccetti, K. Schawinski, N. Z. Scoville, J. Silverman, J. Trump, M. Urry, C. Vignali, N. J. Wright

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 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this paper we report the i, K and 3.6micron 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 2 sources are truly empty fields. Making use of the large number of X-ray sources, we update the "classic locus" of AGN and define a new locus containing 90% of the AGN in the survey with full band luminosity >10^42 erg/s. We present the linear fit between the total i band magnitude and the X-ray flux in the soft and hard band, drawn over 2 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 C and X/O, computed in the hard band, and that 90% of the obscured AGN 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 BH growth at X-ray luminosities of 10^43- 10^44.5 erg/s.

View original: http://arxiv.org/abs/1205.5030