High-Ionization Fe K Emission From Luminous Infrared Galaxies

The Chandra component of the Great Observatories All-Sky Luminous Infrared Galaxy Survey (GOALS) presently contains 44 luminous and ultraluminous infrared galaxies (ULIRGs) with log (L _IR/L _sun) = 11.73-12.57. Omitting 15 obvious active galactic nuclei (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 with 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 a Compton-thick AGN.