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

Abstract

Stellar-gas kinematic misalignments are a transient phenomenon observed in ∼ 11 per cent 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 ∼ 0.1 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 (Evolution and Assembly of GaLaxies and their
Environments) 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 ∼ 5600 galaxies with a stellar mass of M∗ ≥ 109.5 M⊙ 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 = 0.1, 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 ≈ 2 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.

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Identifiers

ORCID for Sandra I. Raimundo: orcid.org/0000-0002-6248-398X

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