Identifying high-redshift active galactic nuclei using X-ray hardness

J. X. Wang, S. Malhotra, J. E. Rhoads, C. A. Norman

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)L109-L112
JournalAstrophysical Journal
Issue number2 II
StatePublished - Sep 10 2004
Externally publishedYes


  • Galaxies: active
  • Galaxies: high-redshift
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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