The Iron Line Background
  We investigate the presence of iron line emission among faint X-ray sources identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep Field North. Individual source spectra are stacked in seven redshift bins over the range z=0.5-4. We find that iron line emission is an ubiquitous property of X-ray sources up to z~3. The measured line strengths are in good agreement with those expected by simple pre-Chandra estimates based on X-ray background synthesis models. The average rest frame equivalent width of the iron line does not show significant changes with redshift. 
Marcella Brusa (1), Roberto Gilli (2) & Andrea Comastri (3) ((1) Max   Planck Institut fuer Extraterrestrische Physik, Garch...