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Recent works have suggested that selection criteria based on MIR colors canbe used to reveal a population of dust-enshrouded, extremely luminous quasarsat z>1. However the X-ray spectral properties of these intriguing sources stillremain largely unexplored. We report on an X-ray spectroscopic study of asample of 44 very bright mid-IR galaxies with extreme mid-IR to optical fluxratios (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 70ks) and, consequently, a sizable fraction of them (~43%) remains undetected inthe 0.5-10 keV band. The vast majority (95%) of the detected sources (23) showan absorption column density NH>10e22 cm-2 and, remarkably, we also find that50% of them can be classified as Type 2 quasars on the basis of theirabsorption properties and X-ray luminosity. Moreover, most of the X-rayundetected sources show extreme mid-IR colors, consistent with being luminousAGN-powered objects, suggesting they might host heavily obscured (possiblyCompton-thick) quasars in X-rays. This demonstrates that our selection criteriaapplied to a wide area survey is very efficient in finding a large number ofType 2 quasars at z > 1. The existence of this class of very powerful, obscuredquasars at high z could have important implications in the context of theformation and cosmological evolution of accreting supermassive black holes andtheir host galaxies.

Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios