Fe K emission from active galaxies in the COSMOS field
  We present a rest-frame spectral stacking analysis of ~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~1. In Type I AGN that have a
typical X-ray luminosity of Lx~1.5e44 erg/s and z~1.6, the cold Fe K at 6.4 keV
is weak (EW~0.05keV), 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, when the X-ray luminosity is restricted
to the range below 3e44 erg/s, or when an intermediate range of Eddington ratio
around 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~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
micron. Given their median redshift of z=2.5, their bolometric luminosity is
likely to reach 10^13 Lsun 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.
