We study the association between active galactic nuclei ( AGN ) and environment at scales of 0.01 - 1 h ^ { -1 } Mpc in the IllustrisTNG simulated universe ( specifically , the TNG100 simulation ) .
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 \lesssim 1.5 ( n = 0.02 h ^ { 3 } Mpc ^ { -3 } at z = 3 ) .
About \sim 10 \% and \sim 1 \% of these BHs live in pairs and multiples , respectively , within 0.1 h ^ { -1 } Mpc scales .
We find that these systems have enhanced likelihood ( up to factors of 3-6 ) of containing high Eddington ratio ( \eta \gtrsim 0.7 ) AGN compared to random pairs and multiples .
Conversely , the likelihood of an AGN to live in 0.1 h ^ { -1 } Mpc scale BH systems is also higher ( by factors \sim 4 for \eta \gtrsim 0.7 ) compared to random pairs and multiples .
We also estimate that \sim 10 \% 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 enhancements in AGN activity are associated with BH pairs and multiples , even at high Eddington ratios .
The enhancement of AGN activity in rich , small-scale ( \lesssim 0.1 h ^ { -1 } Mpc ) environments is therefore likely to be driven by galaxy interactions and mergers .
Nonetheless , the overall percentage of AGN that live in \lesssim 0.1 h ^ { -1 } Mpc scale multiples is still subdominant ( at most \sim 40 \% for the highest Eddington ratio AGN ) .
Furthermore , the associated enhancement in their Eddington ratios of BH systems ( as well as merging BHs ) is only up to factors of \sim 2 - 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 .