The Infrared Counterparts of the Optically Unidentified Chandra Deep Field–South 1 Ms Sources

The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346X-ray sources, of which 59 were not visible on the VLT/FORS1 and theESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using thefirst release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified sixof the twelve such objects that were within the coverage of these IRobservations. The VLT/FORS1 I-band data further confirms that five of these sixobjects are undetected in the optical. The photometric properties of these sixcounterparts are compared against those of the optically brighter counterpartsof Chandra sources in the same field. We found that the location of theseoptically 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 unreddenedAGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed tolie on the E/S0 branch and its extension, suggesting that these X-ray sourcehosts 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 overa wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1mag) is allowed. However, the later interpretation requires fine-tuningextinction together with redshift for these objects individually. If they areindeed AGN/QSOs, the most luminous of them is just barely qualified for being aQSO. Finally, we point out that neither high-redshift (z > 5) star-forminggalaxies nor irregular galaxies at lower redshift can be a viable explanationto the nature of these six counterparts.Comment: Accepted for publication in Ap