THE CHANDRA COSMOS-LEGACY SURVEY: THE z > 3 SAMPLE

Abstract

We present the largest high-redshift (3 < z < 6.85) sample of X-ray-selected active galactic nuclei (AGNs) on a contiguous field, using sources detected in the Chandra COSMOS-Legacy survey. The sample contains 174 sources, 87 with spectroscopic redshift and the other 87 with photometric redshift ( z phot ). In this work, we treat z phot as a probability-weighted sum of contributions, adding to our sample the contribution of sources with z phot  < 3 but z phot probability distribution >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 (log L (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 < logL(2–10 keV) < 44.1 erg s −1 ) from z  ∼ 3 to z  ∼ 4.5. We study the space density evolution dividing our sample into optically classified Type 1 and Type 2 AGNs. At log L (2–10 keV) > 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 log L (2–10 keV) > 44.1 erg s −1 with respect to our data.