IDENTIFICATIONS AND PHOTOMETRIC REDSHIFTS OF THE 2 Ms CHANDRA DEEP FIELD-SOUTH SOURCES

Abstract

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 |Δ z |/(1 + z ) ≈ 1% and an outlier [with |Δ z |/(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.