Hard X‐Ray Luminosities of Multinuclei Infrared Luminous Galaxies Showing a Radio/Far‐Infrared Excess

Abstract

We report the results of hard X-ray observations of four multinuclei merging infrared luminous galaxies (IRLGs). We selected these four sources for their excess of radio-to-far-infrared luminosity ratio compared with starburst galaxies. This excess suggests that activity associated with a supermassive black hole (SMBH) contributes strongly to the IRLGs' bolometric luminosities. Although we expect strong hard X-ray emission from the SMBH-driven activity, the radio-excess multinuclei merging IRLGs show considerably smaller hard X-ray luminosities relative to far-infrared (40-500 μm) and infrared (8-1000 μm) luminosities than active galactic nuclei (AGNs) showing a similar radio excess. This result may demonstrate that emission in the hard X-ray region from SMBH-driven activity in the multinuclei merging IRLGs is severely suppressed compared to a typical spectral energy distribution of SMBH-driven activity in AGNs. If this is a common property of merging IRLGs, without taking this into account, hard X-ray observations underestimate the contribution of SMBH-driven activity to the bolometric luminosities of merging IRLGs.