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

To understand the interactions between active galactic nuclei (AGNs) 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 catalogues from Chandra and XMM-Newton observations, and the far-infrared 70 mu m point-source catalogue from Spitzer-MIPS observations. Although the IRAC [3.6 mu m]-[4.5 mu m] versus [5.8 mu m]-[8.0 mu m] colours of our sample indicate that only similar to 63 per cent of our sources would be classified as AGNs, the ratio of the rest-frame 2-10 keV luminosity to the total infrared luminosity (8-1000 mu m) shows that the entire 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 an 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.