The largest X-ray-selected sample of z>3 AGNs: C-COSMOS and 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 Cosmological Evolution Survey and Chandra Multi-wavelength Project surveys and doubling the previous samples. The sample comprises 209 X-ray-detected AGNs, over a wide range of rest-frame 2-10 keV luminosities log L_X = 43.3-46.0 erg s^-1. X-ray hardness rates show that ∼39 per cent of the sources are highly obscured, N_H > 10²² cm^-2, in agreement with the ∼37 per cent of type-2 AGNs found in our sample based on their optical classification. For ∼26 per cent of objects have mismatched optical and X-ray classifications. Utilizing 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 AGNs (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 AGNs 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.