The Infrared Counterparts of the Optically Unidentified CDF-S 1Ms Sources

Abstract

The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346 X-ray sources, of which 59 were not visible on the VLT/FORS1 and the ESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using the first release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified six of the twelve such objects that were within the coverage of these IR observations. The VLT/FORS1 I-band data further confirms that five of these six objects are undetected in the optical. The photometric properties of these six counterparts are compared against those of the optically brighter counterparts of Chandra sources in the same field. We found that the location of these optically brighter Chandra sources in the near-IR color space was bifurcated, with the color of one branch being consistent with that of E/S0 galaxies at 0 <= z <= 1.5 and the other branch being consistent with that of unreddened AGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed to lie on the E/S0 branch and its extension, suggesting that these X-ray source hosts are mostly luminous E/S0 galaxies (M_V ~ -20 mag in AB system) at 1 <= z <= 2.5. On the other hand, some of them can also be explained by AGN/QSOs over a wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1 mag) is allowed. However, the later interpretation requires fine-tuning extinction together with redshift for these objects individually. If they are indeed AGN/QSOs, the most luminous of them is just barely qualified for being a QSO. Finally, we point out that neither high-redshift (z > 5) star-forming galaxies nor irregular galaxies at lower redshift can be a viable explanation to the nature of these six counterparts.

arXiv commentAccepted for publication in ApJ

References in corpus (14)

• Composite Quasar Spectra From the Sloan Digital Sky Survey
• The Chandra Deep Field South: the 1 Million Second
• The 2dF QSO Redshift Survey - I. The Optical QSO Luminosity Function
• Resolving the extragalactic hard X-ray background
• The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs
• A Classic Type 2 QSO
• The Great Observatories Origins Deep Survey
• New Results from the X-ray and Optical Survey of the Chandra Deep Field South: The 300ks Exposure
• The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-ray Source Population
• Constraints on the Early Formation of Field Elliptical Galaxies
• NICMOS Imaging of the Dusty Microjansky Radio Source VLA J123642+621331 at z = 4.424
• The Chandra Deep Field North Survey. X. X-ray Emission from Very Red Objects
• Detecting high redshift evolved galaxies as the hosts of optically faint hard X-ray sources
• A Simple Prediction on the Surface Density of Galaxies at z ~ 6

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• The Subaru-XMM-Newton Deep Survey (SXDS) VIII.: Multi-wavelength Identification, Optical/NIR Spectroscopic Properties, and Photometric Redshifts of X-ray Sources
• Why Optically--Faint AGN Are Faint: The Spitzer Perspective
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