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

The Chandra COSMOS-Legacy Survey: The z>3 sample
The Chandra COSMOS-Legacy Survey: The z>3 sample
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.
1538-4357
1-16
Marchesi, S.
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Civano, F.
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Salvato, M.
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Shankar, F.
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Comastri, A.
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Elvis, M.
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Lanzuisi, G.
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Trakhtenbrot, B.
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Vignali, C.
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Zamorani, G.
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Allevato, V.
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Brusa, M.
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Fiore, F.
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Gilli, R.
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Griffiths, R.
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Hasinger, G.
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Miyaji, T.
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Schawinski, K.
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Treister, E.
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Urry, C.M.
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Marchesi, S.
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Civano, F.
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Salvato, M.
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Shankar, F.
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Comastri, A.
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Elvis, M.
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Lanzuisi, G.
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Trakhtenbrot, B.
1bbbe48f-d3b9-4bf0-a509-7d006a17bbc7
Vignali, C.
b6521f25-b2ed-4feb-b4c0-debd78f44676
Zamorani, G.
316aede1-9ee4-46e1-8824-69c1e2060308
Allevato, V.
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Brusa, M.
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Fiore, F.
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Gilli, R.
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Griffiths, R.
182e5e39-a441-4969-9748-4788a810a0ad
Hasinger, G.
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Miyaji, T.
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Schawinski, K.
b0386c64-6483-40ab-8537-13e3b0dcde48
Treister, E.
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Urry, C.M.
506a7d1f-d721-4b3a-919d-f5d47647f1ea

Marchesi, S., Civano, F., Salvato, M., Shankar, F., Comastri, A., Elvis, M., Lanzuisi, G., Trakhtenbrot, B., Vignali, C., Zamorani, G., Allevato, V., Brusa, M., Fiore, F., Gilli, R., Griffiths, R., Hasinger, G., Miyaji, T., Schawinski, K., Treister, E. and Urry, C.M. (2016) The Chandra COSMOS-Legacy Survey: The z>3 sample. The Astrophysical Journal, 827 (150), 1-16. (doi:10.3847/0004-637X/827/2/150).

Record type: Article

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.

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Accepted/In Press date: 20 June 2016
e-pub ahead of print date: 19 August 2016
Published date: 22 August 2016
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 400180
URI: http://eprints.soton.ac.uk/id/eprint/400180
ISSN: 1538-4357
PURE UUID: 7af704c8-26dd-406f-9a03-addad6ba07e7

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Date deposited: 12 Sep 2016 15:23
Last modified: 16 Dec 2019 19:44

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Contributors

Author: S. Marchesi
Author: F. Civano
Author: M. Salvato
Author: F. Shankar
Author: A. Comastri
Author: M. Elvis
Author: G. Lanzuisi
Author: B. Trakhtenbrot
Author: C. Vignali
Author: G. Zamorani
Author: V. Allevato
Author: M. Brusa
Author: F. Fiore
Author: R. Gilli
Author: R. Griffiths
Author: G. Hasinger
Author: T. Miyaji
Author: K. Schawinski
Author: E. Treister
Author: C.M. Urry

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