Supermassive black holes grow by accreting gas. The gas has to lose angular momentum to fall inward, and two mechanisms can provide the torque: stellar bars — elongated structures within the galaxy disc that funnel gas toward the center — and galaxy mergers, which disrupt the disc entirely and drive gas inward through tidal forces. Both mechanisms are observed. The question is which one dominates.
Izquierdo-Villalba, Bonoli, and Rosas-Guevara (arXiv 2602.22875, February 2026) use the IllustrisTNG50 cosmological simulation to demonstrate that the answer depends on environment. Isolated galaxies, sitting in quiet regions of the cosmic web, grow their black holes through stellar bars. The disc forms early, remains stable, develops a bar, and the bar drives sustained, coherent accretion over billions of years. Non-isolated galaxies, embedded in denser environments with frequent interactions, grow their black holes through mergers. The mergers arrive before bars can form, or disrupt bars that do form, and provide the dominant fuel delivery.
The critical variable is timing. In isolated systems, bars form early in stable discs and establish the accretion channel before any significant merger occurs. In dense environments, mergers arrive first, trigger rapid black hole growth, and the bars that form later contribute little because the primary growth phase is already over.
Control samples confirm the mechanism: isolated galaxies without bars show no sustained accretion at all — the bar is not optional in quiet environments, it is the only channel. Unbarred galaxies in dense environments still grow their black holes episodically through mergers alone.
The same galaxy, the same black hole mass, the same accretion physics — but the dominant feeding mechanism switches depending on whether the galaxy lives in a void or a group. The physics is internal; the selection is external.