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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Vito, F., Vignali, C., Gilli, R., Comastri, A., Iwasawa, K., Brandt, W.N., Alexander, D.M., Brusa, M., Lehmer, B., Bauer, F.E., Schneider, D.P., Xue, Y.Q. & Luo, B. (2013). The high-redshift (z > 3) active galactic nucleus population in the 4-Ms Chandra Deep Field-South. Monthly notices of the Royal Astronomical Society 428(1): 354-369.

Author(s) from Durham


We present results from a spectral analysis of a sample of high-redshift (z > 3) X-ray-selected active galactic nucleus (AGN) in the 4-Ms Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date. The sample is selected using the most recent spectroscopic and photometric information available in this field. It consists of 34 sources with median redshift z = 3.7, 80 median net counts in the 0.5–7 keV band and median rest-frame absorption-corrected luminosity L2-10 keV ≈ 1.5 × 1044 erg s−1. Spectral analysis for the full sample is presented and the intrinsic column density distribution, corrected for observational biases using spectral simulations, is compared with the expectations of X-ray background (XRB) synthesis models. We find that ≈57 per cent of the sources are highly obscured (NH > 1023 cm−2). Source number counts in the 0.5-2 keV band down to flux F0.5-2 keV ≈ 4 × 10−17 erg s−1 cm−2 are also presented. Our results are consistent with a decline of the AGN space density at z > 3 and suggest that, at those redshifts, the AGN obscured fraction is in agreement with the expectations of XRB synthesis models.