The nature of X-ray absorbed QSOs
A significant population of broad-line z=2 QSOs have heavily absorbed X-ray spectra. Submm observations show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. The radically different star formation properties between the absorbed and unabsorbed QSOs implies that the X-ray absorption is unrelated to the torus invoked in AGN unification schemes. Instead, these objects represent a transitional phase in an evolutionary sequence relating massive black holes and the formation of galaxies. Prior to this phase, the galaxy is rapidly forming its stars, and the growth of the black hole is obscured. After the X-ray absorbed phase, the naked QSO shines brightly, and its host elliptical galaxy is essentially fully formed. The most puzzling question about these objects has always been the nature of the X-ray absorber. I will present a study of the X-ray absorbers based on deep (50-100ks) XMM-Newton spectroscopy, and show that the absorption is due to a dense, ionised wind driven by the QSO, with a kinetic luminosity compatible with the theoretical requirements for producing the M-sigma relation.