Supermassive black holes in merger-free galaxies have higher spins which are preferentially aligned with their host galaxy

TL;DR

This study uses the Horizon-AGN simulation to show that supermassive black holes in merger-free galaxies have higher spins and are more aligned with their host galaxies, highlighting the role of accretion in SMBH evolution.

Contribution

It demonstrates that SMBHs in merger-free galaxies have higher spins and better alignment, emphasizing the importance of accretion over mergers in SMBH growth and galaxy co-evolution.

Findings

Higher SMBH spins in merger-free galaxies (> 5σ significance)

Greater alignment of SMBH spins with host galaxies in merger-free systems

Merger-free SMBHs spend 91% of their lifetime in radio mode feedback since z=2

Abstract

Here we use the Horizon-AGN simulation to test whether the spins of SMBHs in merger-free galaxies are higher. We select samples using an observationally motivated bulge-to-total mass ratio of < 0.1, along with two simulation motivated thresholds selecting galaxies which have not undergone a galaxy merger since z = 2, and those SMBHs with < 10% of their mass due to SMBH mergers. We find higher spins (> 5{\sigma} ) in all three samples compared to the rest of the population. In addition, we find that SMBHs with their growth dominated by BH mergers following galaxy mergers, are less likely to be aligned with their galaxy spin than those that have grown through accretion in the absence of galaxy mergers (3.4{\sigma} ). We discuss the implications this has for the impact of active galactic nuclei (AGN) feedback, finding that merger-free SMBHs spend on average 91% of their lifetimes since z = 2 in a radio mode of feedback (88% for merger-dominated galaxies). Given that previous observational and theoretical works have concluded that merger-free processes dominate SMBH-galaxy co-evolution, our results suggest that this co-evolution could be regulated by radio mode AGN feedback.