Supermassive black hole fueling in IllustrisTNG: Impact of environment

TL;DR

This study uses the IllustrisTNG simulation to analyze how the environment influences supermassive black hole activity, revealing that small-scale interactions boost AGN activity but are not the dominant fueling mechanism overall.

Contribution

It provides new insights into the environmental dependence of AGN activity and quantifies the role of galaxy interactions in fueling supermassive black holes.

Findings

Enhanced AGN activity in small-scale BH pairs and multiples.

Approximately 10% of AGN have detectable companions within 0.1 h^{-1} Mpc.

Mergers and interactions are a minor but significant factor in AGN fueling.

Abstract

We study the association between active galactic nuclei (AGN) and environment at scales of $0.01-1\ h^{-1}$Mpc in the IllustrisTNG (TNG100) simulated universe. We identify supermassive black hole (BH) pairs and multiples within scales of 0.01, 0.1, & 1 $h^{-1}$Mpc and examine their AGN activity in relation to randomly-selected pairs and multiples. The number density of BHs in TNG100 is $n=0.06\,h^3$Mpc$^{-3}$ at $z\lesssim1.5$ ($n=0.02\,h^3$ Mpc$^{-3}$ at $z=3$). About $\sim10$% and $\sim1$% of them live in pairs and multiples, respectively, within 0.1 $h^{-1}$Mpc scales. We find that BH systems have enhanced likelihood (up to factors of 3-6) of containing high Eddington ratio ($\eta\gtrsim0.7$) AGN compared to random pairs and multiples. Conversely, the likelihood of an AGN to live in 0.1$h^{-1}$Mpc scale systems is also higher (by factors $\sim4$ for $\eta\gtrsim0.7$) compared to random pairs and multiples. We also estimate that $\sim10$% of ultra-hard X-ray selected AGN in TNG100 have detectable 2-10 keV AGN companions on $0.1\ h^{-1}$Mpc scales, in agreement with observations. On larger spatial scales ($\sim 1$ $h^{-1}$Mpc), however, no significant enhancement is associated with BH pairs and multiples, even at high Eddington ratios. The enhancement of AGN activity in rich, small-scale ($\lesssim0.1$ $h^{-1}$Mpc) environments is therefore likely to be driven by galaxy interactions and mergers. Nonetheless, the overall percentage of AGN that live in $\lesssim0.1$ $h^{-1}$Mpc scale multiples is still subdominant (at most $\sim40$% for the highest Eddington ratio AGN). Furthermore, the enhancement in Eddington ratios of BH systems(as well as merging BHs) is only up to factors of $\sim2-3$. Thus, our results support the existence of a merger-AGN connection, but they also suggest that mergers and interactions play a relatively minor role in fueling the AGN population as a whole.