Measuring the population of obscured quasars is one of the key issues in understanding the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in the Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this paper, 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 L2-10keV and N-H, we find that 71% +/- 19% (20 out of 28) of the quasars (with intrinsic L2-10 keV > 10(44) ergs s(-1)) are obscured with N-H > 10(22) cm(-2). Taking into account the uncertainties in the measurements of both NH and LX, conservative lower and upper limits for the fraction are 54% (13 out 24) and 84% (31 out 37). In the Chandra Deep Field North, the number is 29%; however, this is mainly due to the redshift incompleteness. We estimate a fraction of similar to 50%-63% after correcting for the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.
Most hard-X-ray-selected quasars in the Chandra Deep Fields are obscured
ROSATI, Piero;
2007
Abstract
Measuring the population of obscured quasars is one of the key issues in understanding the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in the Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this paper, 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 L2-10keV and N-H, we find that 71% +/- 19% (20 out of 28) of the quasars (with intrinsic L2-10 keV > 10(44) ergs s(-1)) are obscured with N-H > 10(22) cm(-2). Taking into account the uncertainties in the measurements of both NH and LX, conservative lower and upper limits for the fraction are 54% (13 out 24) and 84% (31 out 37). In the Chandra Deep Field North, the number is 29%; however, this is mainly due to the redshift incompleteness. We estimate a fraction of similar to 50%-63% after correcting for the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
