Galaxy mergers in EAGLE do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN

Abstract: We investigate the connection between galaxy--galaxy mergers and enhanced black hole (BH) growth using the cosmological hydrodynamical EAGLE simulation. We do this via three methods of analysis, investigating: the merger fraction of AGN, the AGN fraction of merging systems and the AGN fraction of galaxies with close companions. In each case, we find an increased abundance of AGN within merging systems relative to control samples of inactive or isolated galaxies (by up to a factor of $\approx 3$ depending on the analysis method used), confirming that mergers are enhancing BH accretion rates for at least a subset of the galaxy population. The greatest excess of AGN triggered via a merger are found in lower mass ($M_* \sim 10^{10}$~\Msol) gas rich ($f_{\mathrm{gas}} > 0.2$) central galaxies with lower mass BHs ($M_{\mathrm{BH}} \sim 10^{7}$~\Msol) at lower redshifts ($z<1$). We find no enhancement of AGN triggered via mergers in more massive galaxies ($M_* \gtrsim 10^{11}$~\Msol). The enhancement of AGN is not uniform throughout the phases of a merger, and instead peaks within the early \emph{remnants} of merging systems (typically lagging $\approx 300$~Myr post-coalescence of the two galaxies at $z=0.5$). We argue that neither major ($M_{\mathrm{,1}} / M_{\mathrm{,2}} \geq \frac{1}{4}$) nor minor mergers ($\frac{1}{10} < M_{\mathrm{,1}} / M_{\mathrm{,2}} < \frac{1}{4}$) are statistically relevant for enhancing BH masses globally. Whilst at all redshifts the galaxies experiencing a merger have accretion rates that are on average 2--3 times that of isolated galaxies, the majority of mass that is accreted onto BHs occurs outside the periods of a merger. We compute that on average no more than 15\% of a BHs final day mass comes from the enhanced accretion rates triggered via a merger

• The paper demonstrates that galaxy mergers increase AGN activity by up to threefold, especially in gas-rich, lower mass galaxies.
• The paper finds that AGN activity peaks about 300 million years after merger coalescence, highlighting important temporal dynamics.
• The paper reveals that, despite enhanced AGN rates during mergers, these events account for only about 15% of total black hole mass growth.

Analysis of Galaxy Mergers and Black Hole Growth

This paper investigates the interplay between galaxy mergers and the growth of supermassive black holes (BHs) using data-driven insights obtained from cosmological hydrodynamical simulations. The research primarily focuses on whether galaxy mergers significantly contribute to black hole growth and explores the associated augmentation of active galactic nuclei (AGN) activity.

Key Findings

1. AGN Enhancement via Mergers: The study reveals that galaxy mergers do, in fact, enhance BH accretion rates. There is a marked increase in the fraction of galaxies hosting AGNs during merger events, showing up to a threefold rise compared to non-merging or isolated galaxies. This highlights the role of mergers in triggering AGN activity, particularly in more gas-rich, lower mass galaxies at lower redshifts.
2. Mass Dependency of Merger Impact: The research identifies that the most substantial AGN increase due to mergers occurs predominantly in lower mass galaxies, with stellar masses around $10^{10} M_{\odot}$. In contrast, more massive galaxies ($M_* \gtrsim 10^{11} M_{\odot}$) do not exhibit a significant increase in AGN activity during mergers, suggesting a size-dependent effect in merger-driven BH growth.
3. Temporal Characteristics of AGN Activity: Mergers induce non-uniform AGN activity throughout their progression, with the peak activity occurring in the early remnant phase—typically lagging about 300 million years post-coalescence, especially noted at redshift $z=0.5$. This finding implies temporal lags between the merger event and peak AGN activity due to internal dynamical settling or gas inflow delays induced by merger dynamics.
4. Contribution to Black Hole Growth: Despite the enhancement in AGN activity triggered by mergers, the paper posits that major mergers are not statistically relevant in the overall BH growth picture. Although mergers can increase accretion rates to 2-3 times those of isolated galaxies at all redshifts, most BH mass accretion occurs outside these merger periods. The simulation results suggest that the mass accreted during the merger-driven accretion constitutes no more than 15% of the final BH mass.

Theoretical and Practical Implications

• Theoretical Insight: This research underscores the nuanced role of cosmic events like galaxy mergers in BH growth, affirming that while mergers indeed spike AGN activity, they are not the dominant pathway for BH mass accumulation. This challenges the previously held perception that mergers are a significant driver of BH growth, suggesting the necessity to reevaluate BH-galaxy coevolution theories.
• Practical Application: Identifying the prevalence and conditions under which mergers accelerate BH accretion helps astronomers target and prioritize observational campaigns. By focusing on lower mass, gas-rich galaxies at specific merger stages, observational strategies can enhance understanding of AGN phenomena, potentially leading to refining models of galaxy evolution.

Future Directions

The findings open pathways for simulating and observing merger events across a wider parameter space of galaxy types and environments. Future research should aim to refine models that incorporate radiative feedback processes and dynamical friction more accurately to assess their roles in BH mergers and the subsequent AGN activity spikes. Observational verification across various wavelengths, especially in the infrared and X-ray spectra, is necessary to differentiate between merger-induced accretion and secular processes driving AGN during non-merging phases.

Overall, this investigation provides a comprehensive examination of galaxy mergers as minor, yet non-negligible contributors to the cosmic tapestry of black hole accretion and galaxy evolution dynamics, with significant implications for theoretical models and observational practices.