A Classic Type 2 QSO

In the Chandra Deep Field South 1 Ms exposure, we have found, at redshift 3.700 +/- 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 detection limit of 3 10 19 ergs s(-1) cm(-2) Angstrom(-1) and shows narrow lines of Lyalpha, C IV N v He II O vI [O III], and C III]. Their FWHM line widths have a range of similar to700-2300 km s(-1) with an average of approximately similar to1500 km s(-1). The emitting gas is metal-rich (Z similar or equal to 2.5-3 Z(.)). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with N-H greater than or similar to 10(24) cm(-2). An iron Kalpha line with rest-frame energy and equivalent width of similar to6.4 keV and similar to1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2sigma level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kalpha. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is similar to10(45 +/- 0.5) ergs cm(-2) s(-1), which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole powered QSO distribution at high redshift.