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 Nuclear Spectroscopic Telescope Array (NuSTAR). By matching the Spitzer-COSMOS catalog against the NuSTAR-COSMOS catalog, we obtain a sample consisting of a hyperluminous infrared galaxy with log(LIR/L⊙) ≥ 13, 12 ultraluminous infrared galaxies with 12 ≤ log(LIR/L⊙) ≤ 13, and 10 luminous infrared galaxies with 11 ≤ log(LIR/L⊙) ≤ 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 NH > 10 22 cm, including several Compton-thick (NH ∼ 10 24 cm) 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 optically-selected AGNs reported by Netzer (2009), 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. 2015) is briefly discussed. Subject headings: galaxies: active — galaxies: evolution — infrared: galaxies