Fe K emission from active galaxies in the COSMOS field

¹ ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
³ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
⁵ Università di Bologna – Dipartimento di Astronomia, via Ranzani, 1, 40127 Bologna, Italy
⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, USA
⁷ Department of Astrophysical Science, University of Princeton, Peyton Hall 103, Princeton NJ 08544, USA
⁸ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ 85721, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
¹⁰ INAF – IASF Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, 85748 Garching, Germany
¹³ Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, 790-8577 Matsuyama, Japan
¹⁴ University of California Observatories/Lick Observatory, University of California, Santa Cruz CA 95064, USA

Received: 4 October 2011
Accepted: 8 November 2011

Abstract

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 L_X ~ 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.