The XMM deep survey in the CDF-S - II. A 9–20?keV selection of heavily obscured active galaxies at z ?>? 1.7

摘要

We present results on a search of heavily obscured active galaxies z?>?1.7 using the rest-frame 9?20?keV excess for X-ray sources detected in the deep XMM-CDFS survey. Out of?176?sources selected with the conservative detection criteria (>8σ) in the first source catalogue of Ranalli et?al. (in prep.), 46 objects lie in the redshift range of interest with the median redshift? ? 2.5. Their typical rest-frame?10?20?keV luminosity is?1044?erg?s-1, as observed. Among optically faint objects that lack spectroscopic redshift, four were found to be strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption features in their X-ray spectra were used to obtain X-ray spectroscopic redshifts. Using the X-ray colour–colour diagram based on the rest-frame 3?5?keV, 5?9?keV, and?9?20?keV bands, seven objects were selected for their 9?20?keV excess and were found to be strongly absorbed X-ray sources with column density of?NH ≥?0.6?×?1024?cm-2, including two possible Compton thick sources. While they are emitting at quasar luminosity, ~3/4 of the sample objects are found to be absorbed by?NH >?1022?cm-2. A comparison with local AGN at the matched luminosity suggests an increasing trend of the absorbed source fraction for high-luminosity AGN towards high redshifts.