The power of infrared AGN selection in mergers: A theoretical study

Abstract

The role of galaxy mergers in fuelling active galactic nuclei (AGNs) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGNs are often obscured in late-stage mergers. Mid-infrared (IR) colour selection of dust-enshroudedAGNwith, e.g. theWide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly toAGN that dominate emission from the host. To understand how selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of AGN throughout galaxy mergers. Although mid-IR colours closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGNs are missed with common mid-IR selection criteria, even in late-stage, gas-rich majormergers. At z ≲ 0.5, wheremerger signatures and dual nuclei can most easily be detected, we find that amore lenientW1-W2 > 0.5 cut greatly improves completenesswithout significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-colour cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR colour selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (< 3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope.