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

B. Luo, W. N. Brandt, Y. Q. Xue, M. Brusa, D. M. Alexander, F. E. Bauer, A. Comastri, A. Koekemoer, B. D. Lehmer, V. Mainieri, D. A. Rafferty, D. P. Schneider, J. D. Silverman, C. Vignali

Research output: Contribution to journalArticlepeer-review

136 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)560-580
Number of pages21
JournalAstrophysical Journal, Supplement Series
Issue number2
StatePublished - 2010

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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