The power of infrared AGN selection in mergers: a theoretical study
  The role of galaxy mergers in fueling active galactic nuclei (AGN) is still
debated, owing partly to selection effects inherent to studies of the
merger/AGN connection. In particular, luminous AGN are often heavily obscured
in late-stage mergers. Mid-infrared (IR) color selection of dust-enshrouded AGN
with, e.g., the Wide-field Infrared Survey Explorer (WISE) has uncovered large
new populations of obscured AGN. However, this method is sensitive mainly to
AGN that dominate emission from the host. To understand how these selection
biases affect mid-IR studies of the merger/AGN connection, we simulate the
evolution of obscured AGN throughout galaxy mergers. Although mid-IR colors
closely trace luminous, obscured AGN, we show that nearly half of
merger-triggered AGN are missed with common mid-IR selection criteria, even in
late-stage, gas-rich major mergers. At z < 0.5, where merger signatures and
dual nuclei can most easily be detected, we find that a more lenient W1-W2 >
0.5 cut greatly improves completeness without 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-color cut that yields high completeness
and reliability even in starbursting systems. Further, we show that mid-IR
color 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.
