Stellar-gas kinematic misalignments in EAGLE: enhanced SMBH growth in misaligned galaxies

TL;DR

This study uses the EAGLE simulation to show that unstable stellar-gas misalignments in galaxies lead to increased black hole growth, supporting observations that active galactic nuclei are more common in misaligned systems.

Contribution

It demonstrates, through simulation, that unstable stellar-gas misalignments cause enhanced black hole growth, revealing a link between kinematic misalignments and active galactic nuclei.

Findings

Unstable misalignments correlate with increased black hole growth.

Galaxies with long-term counter-rotation show similar black hole activity to aligned galaxies.

Overmassive black holes are more common in galaxies with recent misalignments.

Abstract

Stellar-gas kinematic misalignments are a transient phenomenon observed in $\sim11\%$ of the local galaxy population. According to current models, misaligned gas is expected to lose angular momentum and relax into the galactic plane on timescales of $\sim0.1$ Gyr, driving gas toward the central regions of the galaxy. Recent observational studies have found a higher incidence of active galactic nuclei in misaligned galaxies. We use the EAGLE simulation to explore the connection between stellar-gas misalignments and enhanced central black hole (BH) activity between $0<z<1$. We use a sample of $\sim5600$ galaxies with a stellar mass of $M_{*}\geqslant \mathrm{10^{9.5}}$ M$_\odot$ that feature long-lived stellar-gas alignment, counter-rotation, and unstable misalignments (non-coplanarity). Over time windows of $0.5$ Gyr, we find that galaxies experiencing an unstable misalignment have systematically enhanced BH growth during relaxation. Galaxies with long-term counter-rotation show little difference in BH growth compared to aligned galaxies. We suggest that this enhanced BH growth is driven by loss of angular momentum in unstable misaligned gas discs which is able to drive gas inward toward the vicinity of the BH. At $z\approx0.1$, we find a greater incidence of overmassive BHs in galaxies that have spent a greater fraction of time with unstable stellar-gas kinematic misalignments over the preceding $\approx2$ Gyr compared to control samples of aligned galaxies. In agreement with observations, we conclude that BH activity is enhanced in misaligned systems in EAGLE and suggest that the presence of overmassive BHs may be indicative of a past stellar-gas kinematic misalignment.