Faint high-redshift AGN in theChandradeep field south: the evolution of the AGN luminosity function and black hole demography

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Abstract
Context. We present detection and analysis of faint X-ray sources in the Chandra deep field south (CDFS) using the 4 Ms Chandra observation.Aims. We place constraints on active galactic nuclei (AGN) luminosity functions at z = 3–7, its cosmological evolution, and high-redshift black hole and AGN demography.Methods. We use a new detection algorithm, using the entire three-dimensional data-cube (position and energy), and searching for X-ray counts at the position of high-z galaxies in the GOODS-South survey.Results. This optimized technique results in the identification of 54 AGN at z > 3, 29 of which are new detections. Applying stringent completeness criteria, we derive AGN luminosity functions in the redshift bins 3–4, 4–5, and > 5.8 and for 42.75 < log L(2–10 keV) < 44.5. We combine this data with the luminous AGN luminosity functions from optical surveys and find that the evolution of the high-z, wide luminosity range luminosity function can be modeled by pure luminosity evolution with L ∗ decreasing from 6.6 × 1044 erg/s at z = 3 to L ∗ = 2 × 1044 erg/s at z = 6. We compare the high-z luminosity function with the predictions of theoretical models using galaxy interactions as AGN triggering mechanism. We find that these models are broadly able to reproduce the high-z AGN luminosity functions. Closer agreement is found when we assume a minimum dark matter halo mass for black hole formation and growth. We compare our AGN luminosity functions with galaxy mass functions to derive the high-z AGN duty cycle, using observed Eddington ratio distributions to derive black hole masses. We find that the duty cycle increases with galaxy stellar mass and redshift by a factor of 10–30 from z = 0.25 to z = 4–5. We also report the detection of a large fraction of highly obscured, Compton thick AGN at z > 3 (18-10 +17 %). Their optical counterparts do not show any reddening and we thus conclude that the size of the X-ray absorber is likely smaller than the dust sublimation radius. We finally report the discovery of a highly star-forming galaxy at z = 3.47, arguing that its X-ray luminosity is likely dominated by stellar sources. If confirmed, this would be one of the farthest objects in which stellar sources have been detected in X-rays.
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