Fe K emission from active galaxies in the COSMOS field

K. Iwasawa, V. Mainieri, M. Brusa, A. Comastri, R. Gilli, C. Vignali, G. Hasinger, D. B. Sanders, N. Cappelluti, C. D. Impey, A. Koekemoer, G. Lanzuisi, E. Lusso, A. Merloni, M. Salvato, Y. Taniguchi, J. R. Trump

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37 Scopus citations


We present a rest-frame spectral stacking analysis of ∼1000 X-ray sources detected in the XMM-COSMOS field to investigate the iron-K line properties of active galaxies beyond redshift z ∼ 1. In Type I AGN that have a typical X-ray luminosity of LX ∼ 1.5 × 1044 (erg s-1) and z ∼ 1.6 the cold Fe K at 6.4 keV is weak (EW ∼ 0.05 keV), which agrees with the known trend. In contrast, high-ionization lines of Fe xxv and Fe xxvi are pronounced. These high-ionization Fe K lines appear to have a connection with high accretion rates. While no broad Fe emission is detected in the total spectrum, it might be present, albeit at low significance (∼2σ), when the X-ray luminosity is restricted to the range below 3 × 1044 erg s-1, or when an intermediate range of Eddington ratio around λ ∼ 0.1 is selected. In Type II AGN, both cold and high-ionzation lines become weak with increasing X-ray luminosity. However, we detected strong high-ionization Fe K (EW ∼ 0.3 keV) in the spectrum of objects at z > 2, while we found no 6.4 keV line. We also found that the primary source of the high-ionization Fe K emission are those objects detected with Spitzer-MIPS at 24 μm. Given their median redshift of z ≈ 2.5, their bolometric luminosity is likely to reach 1013 L and the MIPS-detected emission most likely originates from hot dust heated by embedded AGN, probably accreting at high Eddington ratio. These properties match those of rapidly growing black holes in ultra-luminous infrared galaxies at the interesting epoch (z ∼ 2-3) of galaxy formation.

Original languageEnglish (US)
Article numberA86
JournalAstronomy and astrophysics
StatePublished - 2012


  • Galaxies: active
  • Surveys
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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