Evidence for a large fraction of Compton-thick quasars at high redshift

Using mid-infrared and radio selection criteria, we pre-select a sample of candidate high-redshift type 2 quasars in the Subaru XMM-Newton Deep Field (SXDF). To filter out starburst contaminants, we use a Bayesian method to fit the spectral energy distributions (SEDs) between 24-mu m and the B-band, obtain photometric redshifts, and identify the best candidates for high-z type 2 quasars. This leaves us with 12 z(phot) >= 1.7 type 2 quasar candidates in an area approximate to 0.8 deg(2), of which only two have secure X-ray detections. The two detected sources have estimated column densities N-H similar to 2 & 3 x 10(27) m(-2), i.e. heavily obscured but Compton-thin quasars. Given the large bolometric luminosities and redshifts of the undetected objects, the lack of X-ray detections suggests extreme absorbing columns N-H greater than or similar to 10(28) m(-2) are typical. We have found evidence for a population of 'Compton-thick' high-redshift type 2 quasars, at least comparable to, and probably larger than, the type 1 quasar population, although spectroscopic confirmation of their active galactic nuclei nature is important.