The Role of Environment in Triggering Active Galactic Nuclei—Evidence for a Change at <i>z</i>  ∼ 1

Abstract What triggers active galactic nuclei (AGN) in some galaxies and what role this brief period of activity plays in the overall evolution of galaxies are still open questions. This paper explores whether or not the local, on scales of ≈1 Mpc, galaxy density plays a role in triggering AGN when controlling for stellar mass. We consider this question as a function of redshift and AGN selection in the X-ray versus in the IR. We use available density maps within 4.8 deg 2 of the XMM-Large Scale Structure (XMM-LSS) field in the redshift range 0.1 &lt; z &lt; 1.6. Our key result is that the environment may play a role in triggering IR AGN. In particular, at z &gt; 1.2, the incidence of AGN increases in higher-density environments, controlling for stellar mass. However, this dependence reverses at z &lt; 1.2, where the incidence of IR AGN is higher in lower-density environments. By contrast, among X-ray-selected AGN, there is no significant local density dependence. Bootstrapping analysis confirms these conclusions. While these results agree with previous work on both obscured and unobscured AGN, this is the first study to use a consistent methodology across IR and X-ray samples, as well as study IR dependence in this full redshift range. Upcoming large spectroscopic surveys such as the Prime Focus Spectrograph Galaxy Evolution Survey will be critical in further elucidating how the environment affects AGN triggering across different cosmic epochs.