  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
(~4.2sigma) 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 ~0.9-4 keV, i.e., in the
rest-frame energy range of ~5-23 keV. When fixing the photon index to
Gamma=1.8, the measured column density is N_H=1.4^{+0.9}_{-0.5} 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 ~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 accretion. This system may have then been caught
during a major co-eval episode of black hole and stellar mass assembly at early
times. The measure of the number density of heavily obscured AGN at high
redshifts will be crucial to reconstruct the BH/galaxy evolution history from
the beginning.
