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X-ray Spectral Constraints for z~2 Massive Galaxies: The Identification of Reflection-Dominated Active Galactic Nuclei

D. M. Alexander 1 F. E. Bauer 2 W. N. Brandt 3, 4 Emanuele Daddi 5 R. C. Hickox 1, 6 B. D. Lehmer 7, 8 B. Luo 3, 4 Y. Q. Xue 3, 4 M. Young 3, 4 A. Comastri 9 A. del Moro 1 A. C. Fabian 10 R. Gilli 9 A. D. Goulding 1, 6 V. Mainieri 11 J. R. Mullaney 5 M. Paolillo 12 D. A. Rafferty 3 D. P. Schneider 3 O. Shemmer 13 C. Vignali 14
1 Department of Physics
2 Departamento de Astronomía y Astrofísica [Santiago]
3 Department of Astronomy and Astrophysics [PennState]
4 Institute for Gravitation and the Cosmos [PennState]
5 AIM (UMR_7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
6 CfA - Harvard-Smithsonian Center for Astrophysics
7 JHU - Johns Hopkins University
8 GSFC - NASA Goddard Space Flight Center
9 OABO - INAF - Osservatorio Astronomico di Bologna
10 Institute of Astronomy
11 ESO - European Southern Observatory
12 Dipartimento di Scienze Fisiche [Naples]
13 Department of Physics
14 Dipartimento di Astronomia, Universita degli Studi di Bologna
Abstract : We use the 4Ms CDF-S survey to place direct X-ray constraints on the ubiquity of z~2 heavily obscured AGNs in K<22 BzK galaxies. Forty seven of the 222 BzK galaxies in the central region of the CDF-S are detected at X-ray energies, 11 of which have hard X-ray spectral slopes (Gamma<1) indicating the presence of heavily obscured AGN activity. The other 36 X-ray detected BzK galaxies appear to be relatively unobscured AGNs and starburst galaxies; we use X-ray variability analyses over a rest-frame baseline of ~3 years to further confirm the presence of AGN activity in many of these systems. The majority (7 out of 11) of the heavily obscured AGNs have excess IR emission over that expected from star formation (termed "IR-excess galaxies"). However, we find that X-ray detected heavily obscured AGNs only comprise ~25% of the IR-excess galaxy population, which is otherwise composed of relatively unobscured AGNs and starburst galaxies. We find that the typical X-ray spectrum of the heavily obscured AGNs is better characterized by a pure reflection model than an absorbed power-law model, suggesting extreme Compton-thick absorption in some systems. We verify this result by producing a composite rest-frame 2-20 keV spectrum, which has a similar shape as a reflection-dominated X-ray spectrum and reveals an emission feature at rest-frame energy ~6.4 keV, likely to be due to Fe K. These heavily obscured AGNs are likely to be the distant analogs of the reflection-dominated AGNs recently identified at z~0 with >10 keV observatories. On the basis of these analyses we estimate the space density for typical (intrinsic X-ray luminosities of L_X>1E43 erg/s) heavily obscured and Compton-thick AGNs at z~2. Our space-density constraints are conservative lower limits but they are already consistent with the range of predictions from X-ray background models.
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https://hal-cea.archives-ouvertes.fr/cea-00998762
Contributor : Bruno Savelli <>
Submitted on : Monday, June 2, 2014 - 4:35:41 PM
Last modification on : Saturday, April 11, 2020 - 2:16:30 AM

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D. M. Alexander, F. E. Bauer, W. N. Brandt, Emanuele Daddi, R. C. Hickox, et al.. X-ray Spectral Constraints for z~2 Massive Galaxies: The Identification of Reflection-Dominated Active Galactic Nuclei. The Astrophysical Journal, American Astronomical Society, 2011, 738 (1), pp.44. ⟨10.1088/0004-637X/738/1/44⟩. ⟨cea-00998762⟩

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