High-ionization Fe K emission from luminous infrared galaxies
  The Chandra component of the Great Observatories All-Sky LIRG Survey (GOALS)
presently contains 44 luminous and ultraluminous infrared galaxies with log
(Lir/Lsun) = 11.73-12.57. Omitting 15 obvious AGNs, the other galaxies are, on
average, underluminous in the 2-10 keV band by 0.7 dex at a given far-infrared
luminosity, compared to nearby star-forming galaxies with lower star formation
rates. The integrated spectrum of these hard X-ray quiet galaxies shows strong
high-ionization Fe K emission (Fe XXV at 6.7 keV), which is incompatible with
X-ray binaries as its origin. The X-ray quietness and the Fe K feature could be
explained by hot gas produced in a starburst, provided that the accompanying
copious emission from high-mass X-ray binaries is somehow suppressed.
Alternatively, these galaxies may contain deeply embedded supermassive black
holes that power the bulk of their infrared luminosity and only faint
photoionized gas is visible, as seen in some ULIRGs with Compton-thick AGN.
