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. 
E. Kalfountzou, F. Civano, M. Elvis, M. Trichas, P. Green
astro-ph.GA