The Nature of X-ray Absorbed Starburst QSOs and the QSO Evolutionary Scheme

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 Xray
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-anddust-
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.