A Compton-thick active galactic nucleus at z ∼ 5 in the 4 Ms Chandra Deep Field South

R. Gilli, J. Su, C. Norman, C. Vignali, A. Comastri, P. Tozzi, P. Rosati, M. Stiavelli, W. N. Brandt, Y. Q. Xue, B. Luo, M. Castellano, A. Fontana, F. Fiore, V. Mainieri, A. Ptak

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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 (∼4.2σ) 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 ∼ 5, is highly suggestive of Compton-thick absorption. The source X-ray spectrum is seen above the background level in the energy range of ∼0.9-4keV, i.e., in the rest-frame energy range of ∼5-23keV. When fixing the photon index to Γ = 1.8, the measured column density is N H = 1.4+0.9 -0.5 × 1024cm -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 2-10keV band is ∼2.5 × 1044ergs-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.

Original languageEnglish (US)
Article numberL28
JournalAstrophysical Journal Letters
Issue number2 PART II
StatePublished - Apr 1 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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