The Iron Line Background.pdf

a r X i v : a s t r o - p h / 0 5 0 1 5 4 2 v 1 2 5 J a n 2 0 0 5 DRAFT VERSION FEBRUARY 2, 2008 Preprint typeset using LATEX style emulateapj v. 11/12/01 THE IRON LINE BACKGROUND MARCELLA BRUSA Max Planck Institut für extraterrestrische Physik, Giessenbachstrasse 1, D–85748 Garching, Germany ROBERTO GILLI INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I–50125 Firenze, Italy ANDREA COMASTRI INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, I–40127 Bologna, Italy Draft version February 2, 2008 ABSTRACT 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. Subject headings: Surveys – Galaxies: active — X-rays: galaxies – X–rays: general – X–rays:diffuse background 1. INTRODUCTION It has been pointed out already a decade ago (Matt Fabian 1994) that prominent spectral features, and especially the ～ 6.4 keV iron Kα emission line commonly found in Seyferts spec- tra (Nandra Pounds 1994), may lead AGN synthesis models to predict a detectable signature in the spectrum of the cosmic X–ray background (XRB) around a few keV. The integrated contribution of iron line emission in the sources making the XRB has been quantitatively estimated by Gilli et al. (1999), on the basis of a large database of observational re- sults available in the pre-Chandra and XMM-Newton era. The predicted shape of the integrated emission from individual lines was computed following the prescriptions of the XRB synthe- sis model of Comastri et a