Quantifying the Active Galactic Nuclei Fraction in Cosmic Voids via Mid-Infrared Variability

TL;DR

This study uses nearly 12 years of mid-infrared variability data to quantify and compare the active galactic nuclei (AGN) fraction in cosmic voids versus walls, revealing higher AGN activity in voids and uncovering a large population of previously undetected AGNs.

Contribution

It introduces a novel method of using mid-IR variability over a long time baseline to identify AGNs, especially in underdense cosmic regions, expanding detection beyond traditional color-based methods.

Findings

Higher mid-IR variability-AGN fraction in void galaxies compared to wall galaxies.

Identification of a large population of AGNs missed by traditional methods.

Greater prevalence of AGN activity in the most underdense large-scale structures.

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, or AGN). One signature of AGN activity is luminosity variability, which appears in the mid-infrared (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 years of combined multi-epoch 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.