2012A&A...546A..84I

We present results on a search of heavily obscured active galaxies z>1.7 using the rest-frame 9-20keV 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 ∼z≃2.5. Their typical rest-frame 10-20keV luminosity is 10⁴⁴erg/s, 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 3-5keV, 5-9keV, and 9-20keV bands, seven objects were selected for their 9-20keV excess and were found to be strongly absorbed X-ray sources with column density of N_H≥0.6x10²⁴cm^–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 N_H>10²²cm^–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.