  We have performed a spectroscopic study of 7 Type 2 QSOs using the
mid-infrared spectrometer IRS on board the Spitzer Space Telescope. These are
(to our knowledge) the first mid-IR spectra of X-ray selected QSO2s taken. The
objects have been selected according to their high intrinsic luminosities and
column densities in the X-rays. Their spectra strongly differ from template
spectra of Type 2 AGN at lower luminosities. They do not exhibit strong PAH
dust emission features from circum-nuclear star forming regions, typical for
lower luminosity Type 2 Seyfert galaxies or other previously used QSO2
templates, such as the (Ultra)luminous Infrared Galaxy ((U)LIRG) NGC 6240. They
also do not show the ice and silicate absorption features of highly luminous
but deeply embedded compact nuclei seen in some ULIRGs. Instead they reveal a
relatively featureless, rising continuum similar to luminous Type 1 AGN. We
also find evidence for a 10 micron silicate feature in emission. Models of
dusty tori in the AGN unification scenario predict this only for Type 1 AGN.
The ratio of the AGN continuum luminosity at 6 micron to the absorption
corrected 2-10keV X-ray AGN luminosity is very similar to that found in Seyfert
galaxies. X-ray selected QSO2s are thus characterized by powerful AGN in hosts
with a luminosity due to star formation < 1e11 L_sol. The dominance of the AGN
light in the mid-IR spectra of QSO2s together with their flatter spectral
energy distributions (SEDs) places important constraints on models of the
cosmic infrared background and of the star formation history of the universe.
