Identifying high redshift AGNs from X-ray colors

Details Identifying high redshift AGNs from X-ray colors Identifying high redshift AGNs from X-ray colors

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aas...204.4407w&link_type=abstract

American Astronomical Society Meeting 204, #44.07; Bulletin of the American Astronomical Society, Vol. 36, p.732

Astronomy and Astrophysics

Astronomy

Scientific paper

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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