REVEALING A POPULATION OF HEAVILY OBSCURED ACTIVE GALACTIC NUCLEI AT z {approx} 0.5-1 IN THE CHANDRA DEEP FIELD-SOUTH
- Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802 (United States)
- Department of Physics, Durham University, Durham DH1 3LE (United Kingdom)
- Max-Planck-Institut fuer Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching (Germany)
- Pontificia Universidad Catolica de Chile, Departamento de Astronomia y Astrofisica, Casilla 306, Santiago 22 (Chile)
- INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, Bologna (Italy)
- Institute of Astronomy, Madingley Road, Cambridge CB3 0HA (United Kingdom)
- Department of Physics and Astronomy, The Johns Hopkins University, Homewood Campus, Baltimore, MD 21218 (United States)
- Leiden Observatory, Leiden University, Oort Gebouw, P.O. Box 9513 RA, Leiden (Netherlands)
- Dipartimento di Astronomia, Universita di Bologna, Via Ranzani 1, Bologna (Italy)
Heavily obscured (N{sub H} {approx}> 3 x 10{sup 23} cm{sup -2}) active galactic nuclei (AGNs) not detected even in the deepest X-ray surveys are often considered to be comparably numerous to the unobscured and moderately obscured AGNs. Such sources are required to fit the cosmic X-ray background (XRB) emission in the 10-30 keV band. We identify a numerically significant population of heavily obscured AGNs at z {approx} 0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended Chandra Deep Field-South by selecting 242 X-ray undetected objects with infrared-based star-formation rates (SFRs) substantially higher (a factor of 3.2 or more) than their SFRs determined from the UV after correcting for dust extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S region using the 4 Ms Chandra data reveals a hard X-ray signal with an effective power-law photon index of {Gamma} = 0.6{sup +0.3}{sub -0.4}, indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74% {+-} 25% of the selected galaxies host obscured AGNs, within which {approx}95% are heavily obscured and {approx}80% are Compton-thick (CT; N{sub H} > 1.5 x 10{sup 24} cm{sup -2}). The heavily obscured objects in our sample are of moderate intrinsic X-ray luminosity ({approx}(0.9-4) x 10{sup 42} erg s{sup -1} in the 2-10 keV band). The space density of the CT AGNs is (1.6 {+-} 0.5) x 10{sup -4} Mpc{sup -3}. The z {approx} 0.5-1 CT objects studied here are expected to contribute {approx}1% of the total XRB flux in the 10-30 keV band, and they account for {approx}5%-15% of the emission in this energy band expected from all CT AGNs according to population-synthesis models. In the 6-8 keV band, the stacked signal of the 23 heavily obscured candidates accounts for <5% of the unresolved XRB flux, while the unresolved {approx}25% of the XRB in this band can probably be explained by a stacking analysis of the X-ray undetected optical galaxies in the CDF-S (a 2.5{sigma} stacked signal). We discuss prospects to identify such heavily obscured objects using future hard X-ray observatories.
- OSTI ID:
- 21587419
- Journal Information:
- Astrophysical Journal, Vol. 740, Issue 1; Other Information: DOI: 10.1088/0004-637X/740/1/37; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
BACKGROUND RADIATION
COMPUTERIZED SIMULATION
COSMIC PHOTONS
EMISSION
GALAXIES
GALAXY NUCLEI
HARD X RADIATION
MONTE CARLO METHOD
PHOTOMETRY
BOSONS
CALCULATION METHODS
COSMIC RADIATION
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
IONIZING RADIATIONS
MASSLESS PARTICLES
PHOTONS
RADIATIONS
SIMULATION
X RADIATION