Are the fates of supermassive black holes and galaxies determined by individual mergers, or by the properties of their host haloes?

TL;DR

This study uses controlled simulations to explore whether supermassive black hole growth is primarily driven by halo properties or galaxy mergers, finding mergers significantly influence black hole mass and galaxy evolution.

Contribution

It demonstrates that galaxy mergers, rather than halo assembly alone, are crucial for black hole growth and galaxy quenching in the EAGLE simulation.

Findings

Major mergers increase black hole mass by about 4 times.

Mergers reduce halo baryon fraction and quench star formation.

Halo properties alone have little effect on black hole growth without mergers.

Abstract

The fates of massive galaxies are tied to the evolution of their central supermassive black holes (BHs), due to the influence of AGN feedback. Correlations within simulated galaxy populations suggest that the masses of BHs are governed by properties of their host dark matter haloes, such as the binding energy and assembly time, at a given halo mass. However, the full picture must be more complex as galaxy mergers have also been shown to influence the growth of BHs and the impact of AGN. In this study, we investigate this problem through a controlled experiment, using the genetic modification technique to adjust the assembly history of a Milky Way-like galaxy simulated with the EAGLE model. We change the halo assembly time (and hence the binding energy) in the absence of any disruptive merger events, and find little change in the integrated growth of the BH. We attribute this to the angular momentum support provided by a galaxy disc, which reduces the inflow of gas towards the BH and effectively decouples the BH's growth from the halo's properties. Introducing major mergers into the assembly history disrupts the disc, causing the BH to grow $\approx 4\times$ more massive and inject feedback that reduces the halo baryon fraction by a factor of $\approx 2$ and quenches star formation. Merger events appear essential to the diversity in BH masses in EAGLE, and we also show that they increase the halo binding energy; correlations between these quantities may therefore be the result of merger events.