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J. X. Wang (JHU), J. E. Rhoads, S. Malhotra (STScI), C. A. Norman (JHU)
The X-ray color (hardness ratio) can be used to select high redshift active galactic nuclei (AGNs) among the x-ray selected, optically blank candidates. High redshift AGNs should appear soft in X-ray with hardness ratio HR ~ -0.5, even if there is strong absorption with NH up to 1023 cm-2, simply because the absorption redshifts out of the soft X-ray band in the observed frame. Here the X-ray hardness ratio is defined as HR= (H-S)/(H+S), where S and H are the soft and hard band net counts detected by Chandra. High redshift AGNs that are Compton thick (NH >= 1024 cm-2) could have HR ~0.0, however, these should be rare in deep Chandra images, since they have to be ~10 times brighter intrinsicly, which leads to ~ 100 times drop in their space density. All known z > 4 AGNs show soft x-ray colors. Applying the hardness criterion (HR < 0.0) can filter out about 50% of the candidate high redshift AGNs selected from deep Chandra images.
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Bulletin of the American Astronomical Society, 36 #2
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