Role of AGNs in the Luminous Infrared Galaxy phase since z ~ 3

TL;DR

This study investigates the role of active galactic nuclei (AGN) in luminous infrared galaxies since redshift ~3, revealing that all galaxies in the sample likely host AGN, with implications for galaxy evolution and star formation activity.

Contribution

It provides evidence that all 142 galaxies with both X-ray and 70μm detections host AGN, highlighting the interaction between AGN activity and star formation over cosmic time.

Findings

All galaxies show higher X-ray luminosity than pure star-forming galaxies.

70μm detected sources tend to have higher X-ray absorption.

Far-infrared colors suggest star formation mainly powers the infrared emission.

Abstract

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.