The Chandra COSMOS Legacy survey: optical/IR identifications
Finoguenov, A.; Jahnke, K.; Silverman, J. D.; Treister, E.; Salvato, M.; Vignali, C.; Gilli, R.; Ranalli, P.; Comastri, A.; Cappelluti, N.; Brusa, M.; Allevato, V.; Hasinger, G.; Schinnerer, E.; Civano, F.; Elvis, M.; Lanzuisi, G.; Karim, A.; Zamorani, G.; Marchesi, S.; Griffiths, R. E.; Urry, C. M.; Laigle, C.; Smolcic, V.; LaMassa, S. M.; Suh, H.; Miyaji, T.; Schawinski, K.; Trakhtenbrot, B.; Cardamone, C.
United States, Italy, Germany, Mexico, Chile, Finland, France, Sweden, Switzerland, Japan, Croatia
Abstract
We present the catalog of optical and infrared counterparts of the Chandra COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 deg2 of the COSMOS field, combination of 56 new overlapping observations obtained in Cycle 14 with the previous C-COSMOS survey. In this Paper we report the I, K, and 3.6 μm identifications of the 2273 X-ray point sources detected in the new Cycle 14 observations. We use the likelihood ratio technique to derive the association of optical/infrared (IR) counterparts for 97% of the X-ray sources. We also update the information for the 1743 sources detected in C-COSMOS, using new K and 3.6 μm information not available when the C-COSMOS analysis was performed. The final catalog contains 4016 X-ray sources, 97% of which have an optical/IR counterpart and a photometric redshift, while ≃54% of the sources have a spectroscopic redshift. The full catalog, including spectroscopic and photometric redshifts and optical and X-ray properties described here in detail, is available online. We study several X-ray to optical (X/O) properties: with our large statistics we put better constraints on the X/O flux ratio locus, finding a shift toward faint optical magnitudes in both soft and hard X-ray band. We confirm the existence of a correlation between X/O and the the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the analysis of the correlation between the fraction of obscured AGNs and the hard band luminosity, finding a different behavior between the optically and X-ray classified obscured fraction.