Journal Article

Discovery of Compton-thick quasars in the Sloan Digital Sky Survey

C. Vignali, D. M. Alexander, R. Gilli and F. Pozzi

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 404, issue 1, pages 48-59
Published in print May 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI:
Discovery of Compton-thick quasars in the Sloan Digital Sky Survey

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We present new and archival Chandra snapshot (≈10 ks each) observations of 15 optically identified [from the Sloan Digital Sky Survey (SDSS)] Type 2 quasars at z= 0.40–0.73. When combined with existing X-ray data, this work provides complete X-ray coverage for all 25 radio-quiet Type 2 quasars with log L[O III] > 9.28 L from Zakamska et al. Two targets out of 15 were not detected by Chandra and most of the remaining sources are X-ray weak, with nine having less than 10 counts in the 0.5–8 keV band. Low- to moderate-quality spectral analysis was limited to three sources, whose properties are consistent with the presence of column densities in the range NH≈ 1022–1023 cm−2 in the source rest frame. If the [O iii] luminosity is a reliable proxy for the intrinsic X-ray luminosity, the current X-ray data indicate that Compton-thick quasars may hide among ≈65 per cent of the SDSS Type 2 quasar population (LX,meas/LX,[O III] < 0.01); however, since the Type 2 quasar sample is selected on [O iii] luminosity, the estimated Compton-thick quasar fraction may be overestimated. Using archival Spitzer observations, we find that ≈50 per cent of SDSS Type 2 quasars appear to be obscured by Compton-thick material based on both the LX,meas/LX,mid-IR (where mid-IR corresponds to rest frame 12.3 μm) and LX,meas/LX,[O III] ratios. We use this information to provide an estimate of the Compton-thick quasar number density at z≈ 0.3–0.8, which we find is in broad agreement with the expectations from X-ray background models.

Keywords: galaxies: active; galaxies: nuclei; quasars: general

Journal Article.  9979 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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