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arXiv · astro-ph/0611251

Most hard X-ray selected quasars in Chandra Deep Fields are obscured

J. X. Wang·P. Jiang·Z. Y. Zheng·P. Tozzi·C. Norman·R. Giacconi·R. Gilli·G. Hasinger·L. Kewley·V. Mainieri·M. Nonino·P. Rosati·A. Streblyanska·G. Szokoly·A. Zirm·W. Zheng
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Abstract

Measuring the population of obscured quasars is one of the key issues to understand the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this letter we study the population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV band selected samples also filter out most of the X-ray faint sources with too few counts, for which the measurements of N_H and L_X have very large uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20 out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements of both N_H and L_X, conservative lower and upper limits of the fraction are 54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is 29%, however, this is mainly due to the redshift incompleteness. We estimate a fraction of ~ 50% - 63% after correcting the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.

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J. X. Wang, P. Jiang, Z. Y. Zheng, P. Tozzi, C. Norman, R. Giacconi, R. Gilli, G. Hasinger, L. Kewley, V. Mainieri, M. Nonino, P. Rosati, A. Streblyanska, G. Szokoly, A. Zirm, W. Zheng. 2006-11-08. Most hard X-ray selected quasars in Chandra Deep Fields are obscured. https://doi.org/10.1086/510766

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