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

TL;DR

This study probes how local environment influences AGN triggering, distinguishing IR- and X-ray-selected populations over $0.1<z<1.6$ in the XMM-LSS field. Using consistent host-mass binning and adaptive, 1 Mpc-scale density maps, it finds no significant density dependence for X-ray AGN but reveals a redshift-dependent reversal for IR AGN: at $z>1.2$ higher-density regions host more IR AGN, while at $z<1.2$ they favor lower-density environments. Bootstrapping reinforces this reversal for IR and IR-only (obscured) samples, suggesting environment plays a role that evolves with cosmic time and selection method. The results align with some prior work yet emphasize the need for spectroscopic data to confirm the trend and understand the triggering/quenching balance across epochs. These findings inform models of gas accretion, mergers, and secular processes driving AGN activity and galaxy evolution, and motivate upcoming surveys like PFS to improve constraints across cosmic time.

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.

Figures (9)

• Figure 1: The redshift vs. luminosity plots for the X-ray detected AGN ( left) and IR detected AGN ( right) after removing sources matched to host galaxies with stellar masses $\log(\frac{M_*}{M_{\odot}}) < 9.3$ and $\log(\frac{M_*}{M_{\odot}}) > 12.0$, being outside the mass bins used in our study. Unfilled circles in the IR plot represent IR detected AGN without a matched X-ray source.
• Figure 2: The contour lines show the stellar mass versus redshift for all galaxies in the parent catalog at levels of $0.1^{\text{th}}$, $1^{\text{st}}$, $10^{\text{th}}$, $50^{\text{th}}$, and $70^{\text{th}}$ percentile galaxy counts. The points represents the three different AGN samples as indicated in the legend. Histograms on the top and right hand side show the distributions of photometric redshifts and stellar masses of the host galaxies of the X-ray sources (in blue) and IR sources (in bright red), and IR-only (dashed dark red), respectively.
• Figure 3: The AGN percentage in different stellar mass and local density bins. Here the top panels represent the lower redshift bin ($0.1<z<1.2$) whereas the bottom three panels represent the higher redshift bin ($1.2<z<1.6$). In all panels, we apply an additional luminosity cut as indicated in order to have luminosity complete samples per redshift bin.
• Figure 4: Redshifts versus luminosity of the X-ray sources. The gray band illustrates the X-ray AGN sample from Yang2018 who do a similar study to ours (see Section \ref{['sec:discussion']} for further discussion). The color of the bin represents the type of dependence seen in this regime of redshift and AGN luminosity. Blue bins represent regimes of anti-dependence with more dense environments having a lower percentage of galaxies hosting an AGN. Red bins represent regimes of positive correlation between the AGN occurrence and extragalactic environmental density. Gray or pale colored bins represent regimes with no dependence between the AGN occurrence and extragalactic density. The weak coloring of the bins for X-ray detected sources represents the lack of dependence seen in either bin of redshift and luminosity considered in this study. Overlaid on each bin is the significance of the score (see Section \ref{['sec:xray_and_ir_results']})
• Figure 5: Redshifts versus luminosity of the full IR sample. Similar to Figure \ref{['fig:xray_score']}, the color of the bins correspond to the type of dependence seen in a bin of redshift and AGN luminosity with the significance of the score overlaid in each bin. Between the two bins the type of dependence exhibited for IR AGN switches at $z = 1.2$ such that it turns from a positive dependence to a negative one.