The Environmental Switch in Black Hole Feeding: Bar-Driven vs. Merger-Driven Growth in IllustrisTNG50

Abstract

The relative roles of secular disc processes and galaxy interactions in driving the growth of supermassive black hole (SMBH) 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 disc galaxies in isolated and non-isolated 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 discs and establish sustained, coherent accretion phases that regulate late-time SMBH growth. In contrast, in non-isolated 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 discs without bars lack sustained accretion. These results demonstrate an environmental bifurcation in SMBH fueling: mergers act as efficient triggers of early growth in dynamically active systems, while bars regulate prolonged accretion only when they form in quiescent discs. This study provides a unified time-domain framework linking galaxy environment, disc dynamics, and SMBH growth by resolving the temporal ordering of bars, mergers, and accretion.