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, LX(2–10 keV) using the Compton-thin assumption. We compare the LX(2–10 keV) values with broad optical/near-infrared emission-line luminosities and confirm a previous finding by Imanishi &Ueno that the LX(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.