Interacting galaxies in the IllustrisTNG simulations – IX: Mini mergers trigger AGNs in cosmological simulations

ABSTRACT Galaxy mergers are transformative events that can cause gaseous inflows capable of triggering active galactic nuclei (AGNs). Previous studies of AGNs in simulations have mainly focused on major interactions (i.e. between approximately equal mass galaxies), which produce the strongest inflows and, therefore, would be the most likely to trigger AGN activity. However, minor interactions are far more common and may still enhance accretion on to supermassive black holes. We present an analysis of post-merger galaxies from the IllustrisTNG simulation with stellar mass ratios of $\mu >$1:100. We select post-mergers from the TNG50-1 simulation, from redshifts $0\le z< 1$, with stellar masses greater than $10^{10}\,{\rm M}_{\odot }$. We find an AGN excess in post-mergers with a stellar mass ratio as low as 1:40. The AGN excess is mass ratio and luminosity dependent, with 1.2–1.6 times more AGNs found in post-mergers of 1:40 $\le \mu <$1:4 than in matched non-merger controls, and as many as 6 times more AGNs found in major $\mu \ge$1:4 post-mergers. The AGN excess is long lived, between 500 Myr to 1 Gyr post-coalescence, across all of the mass ratio regimes. We demonstrate that the most luminous AGNs in the simulation overwhelmingly occur in either post-mergers or pairs (with $\mu \ge$1:40). Finally, we demonstrate that mini mergers are likely to be overlooked in observational studies due to the weakness of features usually associated with recent merger activity, such as tidal streams and shells, making it challenging to completely account for merger-induced AGN activity even in deep galaxy surveys.