X-RAY SPECTRAL CONSTRAINTS FOR z {approx} 2 MASSIVE GALAXIES: THE IDENTIFICATION OF REFLECTION-DOMINATED ACTIVE GALACTIC NUCLEI

Journal Article · Thu Sep 01 04:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/738/1/44· OSTI ID:21582973

Alexander, D M; Hickox, R C; Del Moro, A; Goulding, A D; Mullaney, J R ^[1]; Bauer, F E ^[2]; Brandt, W N; Luo, B; Xue, Y Q; Young, M; Rafferty, D A; Schneider, D P ^[3]; Daddi, E ^[4]; Lehmer, B D ^[5]; Comastri, A; Gilli, R ^[6]; Fabian, A C ^[7]; Mainieri, V ^[8]; Paolillo, M ^[9]; Shemmer, O ^[10]

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1. Department of Physics, Durham University, Durham DH1 3LE (United Kingdom)
2. Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile)
3. Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802 (United States)
4. Laboratoire AIM, CEA/DSM-CNRS-Universite Paris Diderot, Irfu/SAp, Orme des Merisiers, F-91191 Gif-sur-Yvette (France)
5. The Johns Hopkins University, Homewood Campus, Baltimore, MD 21218 (United States)
6. INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-04127 Bologna (Italy)
7. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA (United Kingdom)
8. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching (Germany)
9. Dipartimento di Scienze Fisiche, Universita di Napoli 'Federico II', Complesso Universitario di Monte S. Angelo V. Cinthia, 9, I-80126 Napoli (Italy)
10. Department of Physics, University of North Texas, Denton, TX 76203 (United States)

We use the 4 Ms Chandra Deep Field-South (CDF-S) survey to place direct constraints on the ubiquity of z {approx} 2 heavily obscured active galactic nuclei (AGNs) in K < 22 BzK-selected galaxies. Forty-seven ({approx}21%) of the 222 BzK-selected galaxies in the central region of the CDF-S are detected at X-ray energies, 11 ({approx}5%) of which have hard X-ray spectral slopes ({Gamma} {approx}< 1), indicating the presence of heavily obscured AGN activity (N{sub H} {approx}> 3 x 10{sup 23} cm{sup -2}). 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 {approx}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 infrared emission over that expected from star formation (termed 'infrared-excess galaxies'). However, we find that X-ray detected heavily obscured AGNs only comprise {approx}25% of the infrared-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 (N{sub H} {approx}> 10{sup 24} cm{sup -2}) 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 {approx}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 {approx} 0 with >10 keV observatories. On the basis of these analyses, we estimate the space density for typical (intrinsic X-ray luminosities of L{sub 2-10keV} {approx}> 10{sup 43} erg s{sup -1}) heavily obscured and Compton-thick AGNs at z {approx} 2. Our space-density constraints are conservative lower limits but they are already consistent with the range of predictions from X-ray background models.

• OSTI ID: 21582973
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 738; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
DENSITY
ELECTROMAGNETIC RADIATION
EMISSION
GALAXIES
GALAXY NUCLEI
INFRARED RADIATION
PHYSICAL PROPERTIES
RADIATIONS
REFLECTION
SPECTRA
X-RAY GALAXIES
X-RAY SPECTRA