The iron line background

We investigate the presence of iron line emission among faint X-ray sources identified in the 1 Ms Chandra Deep Field-South and in the 2 Ms 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 a ubiquitous property of X-ray sources up to z similar or equal to 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.