A Classic Type 2 QSO
Abstract
In the Chandra Deep Field South 1Msec exposure we have found, at redshift 3.700 +- 0.005, the most distant Type 2 AGN 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 3sigma detection limit of ~3 10-19 ergs/s/cm2/AA and shows narrow lines of Lyalpha, CIV, NV, HeII, OVI, [OIII], and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3 Zsolar). In the X-ray spectrum of 130 counts in the 0.5-7 keV band there is evidence for intrinsic absorption with NH > 1024 cm-2. An iron Kalpha 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 2sigma level. If confirmed by our forthcoming XMM observations this would be the highest redshift detection of FeKalpha. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~1045 +- 0.5 ergs/s, which makes our source a classic example of the long sought Type 2 QSOs. 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.
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