The Environmental Switch in Black Hole Feeding: Bar-driven versus Merger-driven Growth in IllustrisTNG50

The relative roles of secular disk processes and galaxy interactions in driving the growth of supermassive black holes (SMBHs) remain unclear. We present a time-resolved, per-galaxy analysis of SMBH mass assembly that explicitly tracks bar formation, merger events, and the environment along the main progenitor branches using the high-resolution IllustrisTNG50 cosmological simulation. We analyze barred and unbarred disk galaxies in isolated and nonisolated environments using physically motivated boundary definitions. We found that SMBH fueling pathways are regulated by the environment through the timing of bar formation relative to mergers. In isolated barred galaxies, stellar bars form early in dynamically cold disks and establish sustained, coherent accretion phases that regulate late-time SMBH growth. In contrast, in nonisolated galaxies, SMBH growth is dominated by early merger-driven accretion episodes, whereas bars form later and contribute weakly to the primary growth phase. Unbarred control samples show that mergers can trigger rapid SMBH growth without bars, but such growth remains episodic, whereas isolated disks without bars lack sustained accretion. These results suggest that SMBH-fueling pathways depend sensitively on environment through the relative timing of bar formation and merger activity. This study provides a unified time-domain framework linking galaxy environment, disk dynamics, and SMBH growth by resolving the temporal ordering of bars, mergers, and accretion.