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

Journal
Date Issued
2016
Author(s)
•
Civano, F.
•
Salvato, M.
•
Shankar, F.
•
•
Elvis, M.
•
•
Trakhtenbrot, B.
•
Vignali, C.
•
Zamorani, G.
•
•
Brusa, M.
•
•
•
Griffiths, R.
•
Hasinger, G.
•
Miyaji, T.
•
Schawinski, K.
•
Treister, E.
•
Urry, C. M.
DOI
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 (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 < 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 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 ∼ 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.
Volume
827
Issue
2
Start page
150
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...827..150M
Rights
open.access
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