科研之友
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摘要
We define a sample of 27 radio-excess AGNs in the Chandra Deep Field-North by selecting galaxies that do not obey the radio/infrared correlation for radio-quiet AGNs and star-forming galaxies. Approximately 60% of these radio-excess AGNs are undetected in X-rays in the 2 Ms Chandra catalog, even at exposures of >= 1 Ms; 25% lack even 2 sigma X-ray detections. The absorbing columns to the faint objects detected in X-rays are 1022 cm(-2) < NH < 10(24) cm(-2); i.e., they are obscured but unlikely to be Compton-thick. Using a local sample of radio-selected AGNs, we show that a low ratio of X-ray to radio emission, as seen in the samples of AGNs weakly detected and undetected in X-rays, is correlated with the viewing angle of the central engine and therefore with obscuration. Our technique can explore the proportion of obscured AGNs in the distant universe; the results reported here for radio-excess objects are consistent with, but at the low end of, the overall theoretical predictions for Compton-thick objects.
- 出版日期2005-11-20
