Identifying high redshift AGNs using X-ray hardness
  The X-ray color (hardness ratio) of optically undetected X-ray sources can be used to distinguish obscured active galactic nuclei (AGNs) at low and intermediate redshift from viable high-redshift (i.e., z>5) AGN candidates. This will help determine the space density, ionizing photon production, and X-ray background contribution of the earliest detectable AGNs. High redshift AGNs should appear soft in X-rays, with hardness ratio HR ~ -0.5, even if there is strong absorption by a hydrogen column density N_H up to 10^23 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 (N_H>~10^24 cm^-2) could have HR~0.0 at z>5. However, these should be rare in deep Chandra images, since they have to be >~10 times brighter intrinsically, which implies >~100 times drop in their space density. Applying the hardness criterion (HR<0.0) can filter out about 50% of the candidate high redshift AGNs selected from deep Chandra images. 
J. X. Wang (JHU, Ustc), S. Malhotra (STScI), J. E. Rhoads (STScI), C.   A. Norman (JHU)