Role of active galactic nuclei in the luminous infrared galaxy phase at z ≤ 3

In order to understand the interactions between active galactic nuclei (AGN) and star formation during the evolution of galaxies, we investigate 142 galaxies detected in both X-ray and 70{\mu}m observations in the COSMOS (Cosmic Evolution Survey) field. All of our data are obtained from the archive, X-ray point source catalogs from Chandra and XMM-Newton observations; far-infrared 70{\mu}m point source catalog from Spitzer-MIPS observations. Although the IRAC [3.6{\mu}m]-[4.5{\mu}m] vs. [5.8{\mu}m]-[8.0{\mu}m] colours of our sample indicate that only ~63% of our sources would be classified as AGN, the ratio of the rest-frame 2-10 keV luminosity to the total infrared luminosity (8-1000{\mu}m) shows that all of the sample has comparatively higher X-ray luminosity than that expected from pure star-forming galaxies, suggesting the presence of an AGN in all of our sources. From the analysis of the X-ray hardness ratio, we find that sources with both 70{\mu}m and X-ray detection tend to have a higher hardness ratio relative to the whole X-ray selected source population, suggesting the presence of more X-ray absorption in the 70{\mu}m detected sources. In addition, we find that the observed far-infrared colours of 70{\mu}m detected sources with and without X-ray emission are similar, suggesting the far-infrared emission could be mainly powered by star formation.