The Largest X-ray Selected Sample of z>3 AGNs: C-COSMOS & ChaMP
  We present results from an analysis of the largest high-redshift (z > 3)
X-ray-selected active galactic nucleus (AGN) sample to date, combining the
Chandra C-COSMOS and ChaMP surveys and doubling the previous samples. The
sample comprises 209 X-ray-detected AGN, over a wide range of rest frame 2-10
keV luminosities logL_{X}=43.3 - 46.0 erg s^{-1}. X-ray hardness rates show
that ~39% of the sources are highly obscured, N_{H}>10^{22} cm^{-2}, in
agreement with the ~37% of type-2 AGN found in our sample based on their
optical classification. For ~26% of objects have mismatched optical and X-ray
classifications. Using the 1/V_{max} method, we confirm that the comoving space
density of all luminosity ranges of AGNs decreases with redshift above z > 3
and up to z ~ 7. With a significant sample of AGN (N=27) at z > 4, it is found
that both source number counts in the 0.5 -2 keV band and comoving space
density are consistent with the expectation of a luminosity dependent density
evolution (LDDE) model at all redshifts, while they exclude the luminosity and
density evolution (LADE) model. The measured comoving space density of type-1
and type-2 AGN shows a constant ratio between the two types at z > 3. Our
results for both AGN types at these redshifts are consistent with the
expectations of LDDE model.
