Fe K emission from active galaxies in the COSMOS field

We present a rest-frame spectral stacking analysis of similar to 1000 X-ray sources detected in the XMM-COSMOS field to investigate the iron-K line properties of active galaxies beyond redshift z similar to 1. In Type I AGN that have a typical X-ray luminosity of L-X similar to 1.5 x 10(44) (erg s(-1)) and z similar to 1.6 the cold Fe K at 6.4 keV is weak (EW similar to 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 (similar to 2 sigma), when the X-ray luminosity is restricted to the range below 3 x 10(44) erg s(-1), or when an intermediate range of Eddington ratio around lambda similar to 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 similar to 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 mu m. Given their median redshift of z similar or equal to 2.5, their bolometric luminosity is likely to reach 10(13) L-circle dot 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 similar to 2-3) of galaxy formation.