Program: GN-2006A-Q-61

There have been several recent claims for a significant population of heavily obscured, luminous type-II QSOs at high z. These are expected in standard models of the X-ray background, and might in principle dominate the accretion budget of the universe. We expect this population to exist based on the orientation-dependent unification schemes which work so well at low redshift. Their identification at high z is, however, based on rest-frame UV emission-lines that do not by themselves provide conclusive proof that these objects are type IIs. The key test is whether broad H-alpha is present, and three X-ray selected type II QSO candidates subjected to this test have all proven in reality to be type Is. We propose to address this issue using further using near-IR spectroscopy to directly measure the H-alpha width in two additional X-ray selected type II candidates, and two selected based on radio/IR fluxes. The results will have a fundamental impact on our understanding of the origin of the X-ray background, the applicability of the unification schemes at high-z, and understanding the luminous QSO population in general.