The Nature of Hard X-Ray (3-24 keV) Detected Luminous Infrared Galaxies in the COSMOS Field

We investigate the nature of far-infrared (70 μm) and hard X-ray (3-24 keV) selected galaxies in the COSMOS field detected with both Spitzer and the Nuclear Spectroscopic Telescope Array (NuSTAR). By matching the Spitzer-COSMOS catalog with the NuSTAR-COSMOS catalog, we obtain a sample consisting of a hyperluminous infrared galaxy with {log}({L}_IR}/{L}_⊙ )≥slant 13, 12 ultraluminous infrared galaxies with 12≤slant {log} ({L}_IR}/{L}_⊙ )≤slant 13, and 10 luminous infrared galaxies with 11≤slant {log} ({L}_IR}/{L}_⊙ )≤slant 12, i.e., 23 Hy/U/LIRGs in total. Using their X-ray hardness ratios, we find that 12 sources are obscured active galactic nuclei (AGNs) with absorption column densities of {N}_{{H}}> {10}²² cm^-2, including several Compton-thick ({N}_{{H}}∼ {10}²⁴ cm^-2) AGN candidates. On the basis of the infrared (60 μm) and intrinsic X-ray luminosities, we examine the relation between star formation (SF) and AGN luminosities of the 23 Hy/U/LIRGs. We find that the correlation is similar to that of the optically selected AGNs reported by Netzer, whereas local, far-infrared selected U/LIRGs show higher SF-to-AGN luminosity ratios than the average of our sample. This result suggests that our Hy/U/LIRGs detected both with Spitzer and NuSTAR are likely situated in a transition epoch between AGN-rising and cold-gas diminishing phases in SF-AGN evolutional sequences. The nature of a Compton-thick AGN candidate newly detected above 8 keV with NuSTAR (ID 245 in Civano et al.) is briefly discussed.