Identifying high-redshift active galactic nuclei 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 NH 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 ≳ 10²⁴ 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 a ≳ 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.