The Chandra COSMOS-Legacy Survey: The z>3 sample

We present the largest high-redshift (3 0 at z > 3. We compute the number counts in the observed 0.5–2 keV band, finding a decline in the number of sources at z > 3 and constraining phenomenological models of the X-ray background. We compute the AGN space density at z > 3 in two different luminosity bins. At higher luminosities (logL(2–10 keV) > 44.1 erg s?1), the space density declines exponentially, dropping by a factor of ~20 from z ~ 3 to z ~ 6. The observed decline is ~80% steeper at lower luminosities (43.55 erg s?1 44.1 erg s?1, unobscured and obscured objects may have different evolution with redshift, with the obscured component being three times higher at z ~ 5. Finally, we compare our space density with predictions of quasar activation merger models, whose calibration is based on optically luminous AGNs. These models significantly overpredict the number of expected AGNs at logL (2–10 keV) > 44.1 erg s?1 with respect to our data.