Quantifying the Active Galactic Nucleus Fraction in Cosmic Voids via Mid-infrared Variability

Abstract Observations and theoretical simulations suggest that the large-scale environment plays a significant role in how galaxies form and evolve and, in particular, whether and when galaxies host an actively accreting supermassive black hole in their center (i.e., an active galactic nucleus; AGN). One signature of AGN activity is luminosity variability, which appears in the mid-IR when circumnuclear dust reprocesses UV and optical photons from the AGN accretion disk. We present here a suite of constraints on the fraction of AGN activity in the most underdense regions of the Universe (cosmic voids) relative to the rest of the Universe (cosmic walls) by using ∼12 yr of combined multiepoch data from AllWISE and NEOWISE to quantify mid-IR variability. We find clear evidence for a larger mid-IR variability−AGN fraction among high- and moderate-luminosity void galaxies compared to their wall counterparts. We also show that mid-IR variability identifies a rather large and unique population of AGNs, the majority of which have eluded detection using more traditional AGN selection methods such as single-epoch mid-IR color selection. The fraction of these newly recovered AGNs is larger among galaxies in voids, suggesting once again more prolific AGN activity in the most underdense large-scale structures of the Universe.