X-ray-Underluminous Active Galactic Nuclei Relative to Broad Emission Lines in Ultraluminous Infrared Galaxies

Abstract

We present X-ray spectra of four ultraluminous infrared galaxies (ULIRGs) with detectable broad near-infrared emission lines produced by active galactic nuclei (AGNs): Mrk 463, PKS 1345+12, IRAS 05189-2524, and IRAS 07598+6508. With the exception of IRAS 07598+6508, high-quality X-ray spectra obtained with XMM or Chandra show modest (30-340 eV) equivalent widths of the 6.4 keV Fe Kα emission line and clear signatures for absorption at a level of 4-33×10²² cm^-2 for the main power-law components from the AGNs. These spectral properties are typical of Compton-thin AGNs, and so we estimate absorption-corrected 2-10 keV X-ray luminosities for the AGNs, L_X(2-10 keV) using the Compton-thin assumption. We compare the L_X(2-10 keV) values with broad optical/near-infrared emission-line luminosities and confirm a previous finding by Imanishi & Ueno that the L_X(2-10 keV) to broad emission-line luminosity ratios in ULIRGs are systematically lower than those of moderately infrared-luminous type 1 AGNs. A comparison of independent energy diagnostic methods suggests that the AGNs are underluminous in the 2-10 keV band with respect to their overall spectral energy distributions, as opposed to the broad emission lines being overluminous. This X-ray underluminosity should be taken into account when using 2-10 keV X-ray data to investigate the energetic contribution from AGNs to the large infrared luminosities of ULIRGs.