The role of environment in triggering AGN -- evidence for a change at $z\sim$1

Jason Reeves, Anna Sajina, Henry Adair, Duncan Farrah, Mark Lacy

First listed 2025-10-02 | Last updated 2025-10-02

Abstract

What triggers AGN in some galaxies and what role does this brief period of activity play in the overall evolution of galaxies are still open questions. This paper explores whether or not the local, on scales of $\approx$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 vs. in the IR. We use available density maps within the 4.8\,sq.deg. XMM-LSS field in the redshift range $0.1 < z < 1.6$. Our key result is that the environment may play a role in triggering IR AGN. In particular, at $z > 1.2$ the incidence of AGN increases in higher density environments, controlling for stellar mass. However, this dependence reverses at $z < 1.2$ 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. Bootstraping 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 (PFS) galaxy evolution survey will be critical in further elucidating how the environment affects AGN triggering across different cosmic epochs.

Short digest

Tests whether 1 Mpc–scale galaxy density modulates AGN incidence in the 4.8 deg² XMM‑LSS field (0.1<z<1.6) while controlling for host stellar mass, comparing IR- and X-ray selections. Finds a redshift-dependent signal for IR-selected AGN: at z>1.2 they are more common in higher-density regions, but at z<1.2 the trend reverses with higher incidence in lower-density environments. X-ray–selected AGN show no significant dependence on local density across the same redshift range, confirmed via bootstrapping. The transition near z≈1.2 implies evolving fueling/obscuration channels where environment plays a stronger role for IR-bright phases of black-hole growth.

Key figures to inspect

• Figure 1: Inspect redshift–luminosity coverage for X-ray vs IR AGN after mass cuts; note IR-only (unfilled) points and where luminosity completeness limits land at each redshift.
• Figure 2: Check host stellar-mass versus redshift contours and the overplotted AGN samples; verify that IR-only and X-ray hosts occupy comparable mass ranges at fixed z (the key control for the incidence comparison).
• Figure 3: Read the AGN fraction heatmaps by stellar mass and local density for low-z vs high-z bins; confirm the sign flip for IR AGN (positive with density at z>1.2, negative at z<1.2) and the muted response for X-ray AGN; note which mass bins drive the trends under the stated luminosity cuts.
• Figure 4: For X-ray AGN, examine the color-coded dependence across redshift–luminosity space and its overlap with Yang+2018; verify the broadly gray/weak colors indicating no local-density dependence within the survey’s sensitivity.