In contradiction to the simple AGN unification schemes, there exists a
significant population of broad line, z~2 QSOs which have heavily absorbed
X-ray spectra. These objects have luminosities and redshifts characteristic of
the sources that produce the bulk of the QSO luminosity in the universe. Our
follow up observations in the submillimetre 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. The most puzzling
question about these objects is the nature of the X-ray absorber. We present
our study of the X-ray absorbers based on deep (50-100ks) XMM-Newton
spectroscopy. The hypothesis of a normal QSO continuum, coupled with a neutral
absorber is strongly rejected. We consider the alternative hypotheses for the
absorber, originating either in the QSO or in the surrounding starburst.
Finally we discuss the implications for QSO/host galaxy formation, in terms of
an evolutionary sequence of star formation and black hole growth. We propose
that both processes occur simultaneously in the gas-and-dust-rich heavily
obscured centres of young galaxies, and that absorbed QSOs form a transitional
stage, between the main obscured growth phase, and the luminous QSO.Comment: To appear in the proceedings of "The X-Ray Universe 2005" conference,
2005 Sept 26-30, El Escorial, Madrid, Spai