Identifications and Photometric Redshifts of the 2 Ms Chandra Deep
  Field-South Sources
  [Abridged] We present reliable multiwavelength identifications and
high-quality photometric redshifts for the 462 X-ray sources in the ~2 Ms
Chandra Deep Field-South. 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 446 (96.5%) of the X-ray sources, with an
expected false-match probability of ~6.2%. 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). The reliability of the photo-z's is
evaluated using the subsample of 220 sources with secure spectroscopic
redshifts. We achieve an accuracy of ~1% [|Delta z|/(1+z)] and an outlier
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 Chandra Deep Field-South.
