2012A&A...537A..86I

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∼1.5x10⁴⁴(erg/s) and z∼1.6 the cold Fe K at 6.4keV is weak (EW∼0.05keV), which agrees with the known trend. In contrast, high-ionization lines of FeXXV and FeXXVI 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 3x10⁴⁴erg/s, 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∼0.3keV) in the spectrum of objects at z>2, while we found no 6.4keV line. We also found that the primary source of the high-ionization Fe K emission are those objects detected with Spitzer-MIPS at 24µm. Given their median redshift of z≃2.5, their bolometric luminosity is likely to reach 10¹³L_☉ 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.