The Synergy between Deep X-ray and Infrared Surveys: AGN and Star Formation Activity

TL;DR

This paper investigates the relationship between infrared and X-ray emissions in AGNs, developing new diagnostics to identify AGN-dominated systems and revealing significant evolution in their infrared to X-ray luminosity ratio over cosmic time.

Contribution

It introduces novel infrared-X-ray diagnostics for AGN identification and measures the evolution of their luminosity ratio from the local universe to high redshift.

Findings

Infrared to X-ray luminosity ratio was ~12 times higher at z<1.5 than today.

New diagnostics effectively identify AGN-dominated systems using 24um and 70um fluxes.

Forthcoming Herschel data will help distinguish between different evolutionary scenarios.

Abstract

We explore the connections between the infrared and X-ray properties of AGNs. Using the well constrained infrared SEDs of a sample of local (i.e., z < 0.1) sample of X-ray AGNs, we develop new diagnostics that exploit 24um and 70um flux densities to identify AGN-dominated systems at z<1.5 and measure their total infrared luminosities. We apply these diagnostics to the X-ray detected AGNs in the Chandra Deep Field South and, in doing so, reveal that the infrared to X-ray luminosity ratio was a factor of ~12 higher in the early Universe compared to today. We explore possible explanations for this dramatic evolution and demonstrate how forthcoming Herschel observations will discriminate between these scenarios and, while doing so, identify potential Compton-thick AGNs. Summary of Mullaney et al., MNRAS, 401, 995 (2010).