The Merger-Triggered Active Galactic Nuclei Contribution to the Ultraluminous Infrared Galaxy Population

TL;DR

This study models the contribution of major galaxy mergers to the population of ultraluminous infrared galaxies (ULIRGs) and finds that mergers dominate local ULIRGs but less so at high redshift, especially for AGN-hosting ULIRGs.

Contribution

It combines an evolving merger-triggered AGN luminosity function with a starburst model to quantify merger contributions to ULIRGs across cosmic time.

Findings

Major mergers account for all local ULIRGs with AGN.

At z ~ 1, mergers contribute less than 12% to ULIRG luminosity density.

High redshift ULIRGs may host heavily obscured, Compton thick AGN.

Abstract

It has long been thought that there is a connection between ultraluminous infrared galaxies (ULIRGs), quasars, and major mergers. Indeed, simulations show that major mergers are capable of triggering massive starbursts and quasars. However, observations by the Herschel Space Observatory suggest that, at least at high redshift, there may not always be a simple causal connection between ULIRGs and mergers. Here, we combine an evolving merger-triggered AGN luminosity function with a merger-triggered starburst model to calculate the maximum contribution of major mergers to the ULIRG population. We find that major mergers can account for the entire local population of ULIRGs hosting AGN and ~25% of the total local ULIRG luminosity density. By z ~ 1, major mergers can no longer account for the luminosity density of ULIRGs hosting AGN and contribute \lesssim 12% of the total ULIRG luminosity density. This drop is likely due to high redshift galaxies being more gas rich and therefore able to achieve high star formation rates through secular evolution. Additionally, we find that major mergers can account for the local population of warm ULIRGs. This suggests that selecting high redshift warm ULIRGs will allow for the identification of high redshift merger-triggered ULIRGs. As major mergers are likely to trigger very highly obscured AGN, a significant fraction of the high redshift warm ULIRG population may host Compton thick AGN.