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

By Alejo Martínez-sansigre, Steve Rawlings, David G. Bonfield, Silvia Mateos, Chris Simpson, Mike Watson, Omar Almaini, Sébastien Foucaud, Kazuhiro Sekiguchi and Yoshihiro Ueda


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-µm and B-band, obtain photometric redshifts, and identify the best candidates for high-z type-2 quasars. This leaves us with 12 zphot � 1.7 type-2 quasar candidates in an area ∼ 0.8 deg 2, of which only two have secure X-ray detections. The two detected sources have estimated column densities NH ∼2 & 3 × 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 NH ∼> 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 AGN nature is important. Key words: quasars:general-galaxies:nuclei-galaxies:active-X-rays:galaxies

Year: 2007
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