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

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 4Ms Chandra observations show a significant (similar to 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 similar to 5, is highly suggestive of Compton-thick absorption. The source X-ray spectrum is seen above the background level in the energy range of similar to 0.9-4 keV, i.e., in the rest-frame energy range of similar to 5-23 keV. When fixing the photon index to Gamma = 1.8, the measured column density is N-H = 1.4(-0.5)(+0.9) x 10(24) cm(-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 similar to 2.5 x 10(44) erg s(-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.