  We report the detection of relativistically broadened iron K alpha emission
in the X-ray spectra of AGN detected in the 4Ms CDF-S. Using the Bayesian X-ray
analysis (BXA) package, we fit 199 hard band (2-7 keV) selected sources in the
redshift range z=0.5--4 with three models: (i) an absorbed power-law, (ii) the
first model plus a narrow reflection component, and (iii) the second model with
an additional relativistic broadened reflection. The Bayesian evidence for the
full sample of sources selects the model with the additional broad component as
being 10^5 times more probable to describe the data better than the second
model. For the two brightest sources in our sample, CID 190 (z=0.734) and CID
104 (z=0.543), BXA reveals the relativistic signatures in the individual
spectra. We estimate the fraction of sources containing a broad component to be
54^{+35}_{-37}% (107/199 sources). Considering that the low signal-to-noise
ratio of some spectra prevents the detection of the broad iron K alpha line, we
infer an intrinsic fraction with broad emission of around two thirds. The
detection of relativistic signatures in the X-ray spectra of these sources
suggests that they are powered by a radiatively efficient accretion disk.
Preliminary evidence is found that the spin of the black hole is high, with a
maximally spinning Kerr BH model (a=1) providing a significantly better fit
than a Schwarzschild model (a=0). Our analysis demonstrate the potential of
X-ray spectroscopy to measure this key parameter in typical SMBH systems at the
peak of BH growth.
