Photometric Redshift of X‐Ray Sources in the Chandra Deep Field–South

Based on the photometry of 10 near-UV, optical, and near-infrared bands ofthe Chandra Deep Field South, we estimate the photometric redshifts for 342X-ray sources, which constitute ~99% of all the detected X-ray sources in thefield. The models of spectral energy distribution are based on galaxies and acombination of power-law continuum and emission lines. Color information isuseful for source classifications: Type-I AGN show non-thermal spectralfeatures that are distinctive from galaxies and Type-II AGN. The hardness ratioin X-ray and the X-ray-to-optical flux ratio are also useful discriminators.Using rudimentary color separation techniques, we are able to further refineour photometric redshift estimations. Among these sources, 137 have reliablespectroscopic redshifts, which we use to verify the accuracy of photometricredshifts and to modify the model inputs. The average relative dispersion inredshift distribution is ~8%, among the most accurate for photometric surveys.The high reliability of our results is attributable to the high quality andbroad coverage of data as well as the applications of several independentmethods and a careful evaluation of every source. We apply our redshiftestimations to study the effect of redshift on broadband colors and to studythe redshift distribution of AGN. Our results show that both the hardness ratioand U-K color decline with redshift, which may be the result of a K-correction.The number of Type-II AGN declines significantly at z>2 and that of galaxiesdeclines at z>1. However, the distribution of Type-I AGN exhibits less redshiftdependence. As well, we observe a significant peak in the redshift distributionat z=0.6. We demonstrate that our photometric redshift estimation produces areliable database for the study of X-ray luminosity of galaxies and AGN.Comment: 40 pages, 11 figures. Accepted for publication in the Astrophysical Journa