标题： 光学未识别的 CDF-S 1Ms 源的红外对应体 显示英文标题

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

摘要： 钱德拉深场南（CDF-S）1Ms曝光产生了346个X射线源的目录，其中59个在VLT/FORS1和ESO-MPI/WFI深度R波段图像中不可见，极限值为R_{薇歌}=26.1–26.7星等。利用CDF-S上的ESO VLT/ISAAC JHK_s数据的第一版发布，我们识别出这十二个对象中有六个位于这些红外观测的覆盖范围内。VLT/FORS1 I波段数据进一步确认了这六个对象中有五个在光学上未被检测到。我们将这六个对应体的测光性质与同一区域内Chandra源更亮的光学对应体进行了比较。我们发现这些更亮的Chandra源在近红外颜色空间中的位置分为两支，一支的颜色与0 <= z <= 1.5范围内的E/S0星系一致，另一支的颜色与未红化的AGN/QSOs在0 <= z <= 3.5范围内的颜色一致。我们所识别的六个对应体似乎位于E/S0分支及其延伸线上，表明这些X射线源宿主主要是亮度较高的E/S0星系（AB系统中M_V ~ -20星等）在1 <= z <= 2.5范围内。另一方面，如果允许内部消光范围（A_V=0–1星等），它们也可以通过一个广泛的红移范围（0 <= z <= 5）内的AGN/QSOs来解释。然而，这种解释需要为这些天体单独精细调整消光和红移。如果它们确实是AGN/QSOs，那么最亮的一个勉强可以被归类为QSO。最后，我们指出，高红移（z > 5）的星形成星系以及低红移的不规则星系都不能作为这六个对应体本质的合理解释。 显示英文摘要

摘要： 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.