Revealing X-ray obscured quasars in SWIRE sources with extreme
  mid-IR/optical flux ratios
  Recent works have suggested that selection criteria based on MIR colors can
be used to reveal a population of dust-enshrouded, extremely luminous quasars
at z>1. However the X-ray spectral properties of these intriguing sources still
remain largely unexplored. We report on an X-ray spectroscopic study of a
sample of 44 very bright mid-IR galaxies with extreme mid-IR to optical flux
ratios (MIR/O>2000). The X-ray coverage of the sample is highly inhomogeneous
(from snap-shot 5 ks Chandra observations to medium-deep XMM exposures of 70
ks) and, consequently, a sizable fraction of them (~43%) remains undetected in
the 0.5-10 keV band. The vast majority (95%) of the detected sources (23) show
an absorption column density NH>10e22 cm-2 and, remarkably, we also find that
50% of them can be classified as Type 2 quasars on the basis of their
absorption properties and X-ray luminosity. Moreover, most of the X-ray
undetected sources show extreme mid-IR colors, consistent with being luminous
AGN-powered objects, suggesting they might host heavily obscured (possibly
Compton-thick) quasars in X-rays. This demonstrates that our selection criteria
applied to a wide area survey is very efficient in finding a large number of
Type 2 quasars at z > 1. The existence of this class of very powerful, obscured
quasars at high z could have important implications in the context of the
formation and cosmological evolution of accreting supermassive black holes and
their host galaxies.
