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

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 (>8 sigma) 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) over tilde similar or equal to 2.5. Their typical rest-frame 10-20 keV luminosity is 10(44) 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 N-H >= 0.6 x 10(24) cm(-2), including two possible Compton thick sources. While they are emitting at quasar luminosity, similar to 3/4 of the sample objects are found to be absorbed by N-H > 10(22) 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.