A COMPTON-THICK ACTIVE GALACTIC NUCLEUS AT z {approx} 5 IN THE 4 Ms CHANDRA DEEP FIELD SOUTH

Journal Article · Fri Apr 01 00:00:00 EDT 2011 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/730/2/L28· OSTI ID:21560413

Gilli, R; Comastri, A ^[1]; Su, J; Norman, C ^[2]; Vignali, C ^[3]; Tozzi, P ^[4]; Rosati, P; Mainieri, V ^[5]; Stiavelli, M ^[6]; Brandt, W N; Xue, Y Q; Luo, B ^[7]; Castellano, M; Fontana, A; Fiore, F ^[8]; Ptak, A ^[9]

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1. INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna (Italy)
2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)
3. Dipartimento di Astronomia, Universita degli Studi di Bologna, Via Ranzani 1, 40127 Bologna (Italy)
4. INAF-Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131, Trieste (Italy)
5. European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei Muenchen (Germany)
6. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
7. Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802 (United States)
8. INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Italy)
9. NASA/GSFC, Greenbelt, MD 20771 (United States)

We report the discovery of a Compton-thick active galactic nucleus (AGN) at z = 4.76 in the 4 Ms Chandra Deep Field South. This object was selected as a V-band dropout in HST/ACS images and previously recognized as an AGN from optical spectroscopy. The 4 Ms Chandra observations show a significant ({approx}4.2{sigma}) X-ray detection at the V-band dropout position. The X-ray source displays a hardness ratio of HR = 0.23 {+-} 0.24, which, for a source at z {approx} 5, is highly suggestive of Compton-thick absorption. The source X-ray spectrum is seen above the background level in the energy range of {approx}0.9-4 keV, i.e., in the rest-frame energy range of {approx}5-23 keV. When fixing the photon index to {Gamma} = 1.8, the measured column density is N{sub H} = 1.4{sup +0.9}{sub -0.5} x 10{sup 24} cm{sup -2}, which is Compton thick. To our knowledge, this is the most distant heavily obscured AGN, confirmed by X-ray spectral analysis, discovered so far. The intrinsic (de-absorbed), rest-frame luminosity in the 2-10 keV band is {approx}2.5 x 10{sup 44} erg s{sup -1}, which places this object among type-2 quasars. The spectral energy distribution shows that massive star formation is associated with obscured black hole (BH) accretion. This system may have then been caught during a major coeval episode of BH and stellar mass assembly at early times. The measure of the number density of heavily obscured AGN at high redshifts will be crucial to reconstructing the BH/galaxy evolution history from the beginning.

• OSTI ID: 21560413
• Journal Information: Astrophysical Journal Letters, Vol. 730, Issue 2; Other Information: DOI: 10.1088/2041-8205/730/2/L28; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ENERGY SPECTRA
GALAXY NUCLEI
X-RAY DETECTION
X-RAY GALAXIES
X-RAY SPECTRA
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
DETECTION
GALAXIES
RADIATION DETECTION
SPECTRA