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 (logL(2-10 keV) > 44.1 erg s-1), the space density declines exponentially, dropping by a factor of â¼20 from z â¼ 3 to z â¼ 6. The observed decline is â¼80% steeper at lower luminosities (43.55 erg s-1 < logL(2-10 keV) < 44.1 erg s-1) from z â¼ 3 to z â¼ 4.5. We study the space density evolution dividing our sample into optically classified Type 1 and Type 2 AGNs. At logL(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 â¼ 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.
|Titolo:||THE CHANDRA COSMOS-LEGACY SURVEY: The z > 3 SAMPLE|
|Data di pubblicazione:||2016|
|Parole Chiave:||galaxies: active; galaxies: evolution; X-rays: galaxies; Astronomy and Astrophysics; Space and Planetary Science|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.3847/0004-637X/827/2/150|
|Appare nelle tipologie:||1.1 Articolo in rivista|