We present a rest-frame spectral stacking analysis of \sim 1000 X-ray sources detected in the XMM-COSMOS field in order to investigate the iron K line properties of active galaxies beyond redshift z \sim 1 .
In Type I AGN that have a typical X-ray luminosity of L _ { X } \sim 1.5 \times 10 ^ { 44 } ( erg s ^ { -1 } ) and z \sim 1.6 , the cold Fe K at 6.4 keV is weak ( EW \sim 0.05 keV ) , in agreement 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 ( \sim 2 \sigma ) , when the X-ray luminosity is restricted to the range below 3 \times 10 ^ { 44 } erg s ^ { -1 } , or when an intermediate range of Eddington ratio around \lambda \sim 0.1 is selected .
In Type II AGN , both cold and high-ionzation lines become weak with increasing X-ray luminosity .
However , strong high-ionization Fe K ( EW \sim 0.3 keV ) is detected in the spectrum of objects at z > 2 , while no 6.4 keV line is found .
It is then found that the primary source of the high-ionization Fe K emission is those objects detected with Spitzer-MIPS at 24 \mu m. Given their median redshift of z \simeq 2.5 , their bolometric luminosity is likely to reach 10 ^ { 13 } L _ { \odot } 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 \sim 2 - 3 ) of galaxy formation .