Identifications and photometric redshifts of the 2 Ms chandra deep field-south sources

We present reliable multiwavelength identifications and high-quality photometric redshifts for the 462 X-ray sources in the 2 Ms Chandra Deep Field-South (CDF-S) survey. Source identifications are carried out using deep optical-to-radio multiwavelength catalogs, and are then combined to create lists of primary and secondary counterparts for the X-ray sources. We identified reliable counterparts for 442 (95.7%) of the X-ray sources, with an expected false-match probability of 6.2%; we also selected four additional likely counterparts. The majority of the other 16 X-ray sources appear to be off-nuclear sources, sources associated with galaxy groups and clusters, high-redshift active galactic nuclei (AGNs), or spurious X-ray sources. A likelihood-ratio method is used for source matching, which effectively reduces the false-match probability at faint magnitudes compared to a simple error-circle matching method. We construct a master photometric catalog for the identified X-ray sources including up to 42 bands of UV-to-infrared data, and then calculate their photometric redshifts (photo-z's). High accuracy in the derived photo-z's is accomplished owing to (1) the up-to-date photometric data covering the full spectral energy distributions (SEDs) of the X-ray sources, (2) more accurate photometric data as a result of source deblending for 10% of the sources in the infrared bands and a few percent in the optical and near-infrared bands, (3) a set of 265 galaxy, AGN, and galaxy/AGN hybrid templates carefully constructed to best represent all possible SEDs, (4) the Zurich Extragalactic Bayesian Redshift Analyzer used to derive the photo-z's, which corrects the SED templates to best represent the SEDs of real sources at different redshifts and thus improves the photo-z quality. The reliability of the photo-z's is evaluated using the subsample of 220 sources with secure spectroscopic redshifts. We achieve an accuracy of |Δz|/(1 + z) 1% and an outlier [with |Δz|/(1 + z)>0.15] fraction of 1.4% for sources with spectroscopic redshifts. We performed blind tests to derive a more realistic estimate of the photo-z quality for sources without spectroscopic redshifts. We expect there are 9% outliers for the relatively brighter sources (R ≲ 26), and the outlier fraction will increase to 15%-25% for the fainter sources (R ≳ 26). The typical photo-z accuracy is 6%-7%. The outlier fraction and photo-z accuracy do not appear to have a redshift dependence (for z 0-4). These photo-z's appear to be the best obtained so far for faint X-ray sources, and they have been significantly (≳50%) improved compared to previous estimates of the photo-z's for the X-ray sources in the 2 Ms Chandra Deep Field-North and 1 Ms CDF-S.