Interacting galaxies in the IllustrisTNG simulations - IX: Mini mergers trigger AGN in cosmological simulations

TL;DR

This study uses the high-resolution TNG50-1 simulation to quantify how galaxy mergers across a wide mass-ratio range $1:100\le\mu<1:1$ trigger AGN activity. By pairing post-merger galaxies with five carefully matched non-merger controls, it finds a statistically significant AGN excess starting at $\mu>1:40$, strongest for major mergers and persisting for 0.5–2 Gyr, with longer durations at lower luminosities. The analysis reveals that mini and minor mergers contribute substantially to the AGN population, including a large fraction of the most luminous AGN residing in interacting systems, while many mini mergers may lack obvious observational merger signatures. The results emphasize the need to include lower-mass-ratio interactions when interpreting AGN triggering in both simulations and observations, and highlight resolution effects as a key caveat in connecting merger dynamics to SMBH fueling.

Abstract

Galaxy mergers are transformative events that can cause gaseous inflows capable of triggering active galactic nuclei (AGN). Previous studies of AGN 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 onto supermassive black holes. We present an analysis of post-merger galaxies from the IllustrisTNG simulation with stellar mass ratios of $μ>$1:100. We select post-mergers from the TNG50-1 simulation, from redshifts $0\leq z< 1$, with stellar masses greater than $10^{10}M_{\odot}$. 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 AGN found in post-mergers of 1:40$\leq μ< $1:4 than in matched non-merger controls, and as many as 6 times more AGN found in major $μ\geq$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 AGN in the simulation overwhelmingly occur in either post-mergers or pairs (with $μ\geq $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.

Figures (13)

• Figure 1: The distribution of the TNG50-1 post-merger mass ratios for mergers meeting the criteria of: a stellar mass of at least $10^{10} M_{\odot}$, z<1, and the merger is between subhalos originating from separate FOF groups. The vertical dashed lines delineate the three mass ratio regimes considered in this paper, namely major ($\mu \geq$ 1:4), minor (1:10 $\leq \mu <$ 1:4) and mini (1:100 $\leq \mu <$ 1:10).
• Figure 2: Top panel: the SMBH masses and luminosities for all TNG50-1 galaxies meeting our redshift and stellar mass criteria, i.e. stellar mass over $10^{10} M_{\odot}$ and $z<1$. Bottom panel: the luminosity function for galaxies in our sample. Different AGN luminosity regimes are highlighted in the dashed lines, and the percentage contribution to the overall population is shown in the legend. The most common (mode) accretion rate is shown in the solid line.
• Figure 3: The distribution of the successfully control-matched post-mergers in mass ratio (first panel from the top), stellar mass (second panel), nearest neighbour distance $\mathrm{r}_1$ (third panel), and the number of neighbours $\mathrm{N}_2$ (fourth panel). In the first panel from the top, the dashed blue line shows the post-merger distribution before control matching. In the following panels, the dashed yellow line shows the distribution for the matched controls. In all panels, the matched post-mergers are shown in solid blue bars.
• Figure 4: The probability that the distribution of the post-merger SMBH accretion rates (SMBHARs) are sampled from the same population as the matched control SMBH accretion rates, according to a KS-test. The KS-test is performed on post-mergers (and associated controls) with a mass ratio +/- 0.1 dex from the corresponding mass ratio of the x-axis. Post-mergers and controls have statistically indistinguishable SMBHARs at mass ratios below 1:40, and are statistically distinct for mass ratios above 1:40.
• Figure 5: The offset of the post-merger SMBHAR compared with the median SMBHAR of the matched controls. Shown in grey is the expected width of $\Delta \dot M_{BH}$ due to SMBHAR stochasticity, demonstrated through a control matched non-merger population. Shown in blue are the post-mergers in three bins of mass ratio: mini mergers from 1:40 - 1:10 (left) , minor mergers from 1:10 - 1:4 (centre) , and major mergers 1:4 - 1:1 (right). All three populations show enhanced SMBHAR compared with controls, with the strongest enhancement found in major mergers.