2002ApJ...571..218N

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 σ detection limit of ∼3x10^–19 ergs.s^–1.cm^–2.Å^–1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ∼700-2300 km.s^–1 with an average of approximately ∼1500 km.s^–1. The emitting gas is metal-rich (Z≃2.5-3Z_☉). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with N_H≳10²⁴ cm^–2. An iron Kα line with rest-frame energy and equivalent width of ∼6.4 keV and ∼1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ∼10^45±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.