The optical/near-infrared broad-line emission and hard X-ray continuum of active galactic nuclei and ultraluminous infrared galaxies

Abstract

We compare the luminosities of the hard X-ray and optical/near-infrared broad-line emission from active galactic nuclei (AGNs) and ultraluminous infrared galaxies (ULIRGs). Both types of emission are thought to be good indicators of activity driven by supermassive black holes. We selected objects for which both X-ray and broad-line data are available, and, additionally, for which we can obtain information about the far-infrared and/or morphological properties. We find that, although the hard X-ray to broad Hα luminosity ratio L_X/L_(broad Hα) is relatively constant in nearly all of the objects examined, there exist some objects showing a distinctly small L_X/L_(broad Hα) ratio. These small L_X/L_(broad Hα) objects are thought to be undergoing a merger based on 60 mum-to-optical flux ratio and/or double nuclei morphology. This result could give us important information about the issue of the energy source of merging ULIRGs.