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.

Figures (9)

• Figure 1: Face-on stellar light composite images of the 16 TNG50-1 galaxies analyzed in this work at $z=0$ (snapshot 99). Rows correspond to barred isolated, barred non-isolated, unbarred isolated, and unbarred non-isolated systems, respectively. The images are synthetic JWST-like stellar composites constructed using the F070W, F115W, and F200W filters. All galaxies are shown on a common physical scale and orientation to enable direct structural comparison.
• Figure 2: Black hole growth histories of barred isolated galaxies. Shown are the cumulative black hole mass growth (solid blue curves) and instantaneous accretion rates (purple curves) for four barred systems evolving in dynamically quiet environments. Vertical dashed lines mark the adopted boundary time, and dotted lines indicate the epoch of bar onset. Merger markers denote the timing of major and minor interactions. In these systems, major mergers occur predominantly at early times, after which long-lived stellar bars develop. In several cases, substantial black hole growth continues following bar formation, with extended periods of moderate accretion characteristic of secular inflow. However, the relative contribution of post-bar growth varies among systems, indicating that bar formation does not uniformly dominate the total black hole mass assembly even in isolated environments.
• Figure 3: Black hole growth histories of barred non--isolated galaxies. Shown are the cumulative black hole mass growth (solid blue curves) and instantaneous accretion rates (purple curves) for four barred systems evolving in dynamically perturbed environments. Vertical dashed lines mark the adopted boundary time, and dotted lines indicate the epoch of bar onset. Merger markers denote the timing of major and minor interactions. In these systems, substantial black hole mass assembly is closely associated with merger-- or interaction--driven accretion episodes that occur prior to or near the time of bar formation. The cumulative growth curves exhibit step--like increases during these early epochs, while the accretion histories are characterized by short--lived, high--amplitude bursts rather than sustained inflow. Although bars form subsequently, the primary phase of black hole growth is largely established by merger activity in dynamically active environments.
• Figure 4: Black hole growth histories of unbarred isolated galaxies. Shown are the cumulative black hole mass growth (solid blue curves) and instantaneous accretion rates (purple curves) for four unbarred systems evolving in dynamically quiescent environments. Vertical dashed lines mark the adopted boundary time, while merger markers indicate the timing of major and minor interactions. In the absence of a long-lived stellar bar, black hole growth generally proceeds without a sustained, high-amplitude secular phase. Accretion remains low to moderate in amplitude and irregular in time, although individual systems can exhibit discrete late-time growth episodes. These histories therefore reflect predominantly secular or stochastic fueling in isolated environments rather than prolonged bar-driven regulation.
• Figure 5: Black hole growth histories of unbarred non--isolated galaxies. Shown are the cumulative black hole mass growth and instantaneous accretion rates for four unbarred systems evolving in dynamically perturbed environments. Black hole growth proceeds through short--lived, high--amplitude accretion episodes associated with merger or interaction events, producing step--like increases in the cumulative mass. Although accretion is episodic rather than smoothly sustained, significant mass assembly often occurs near or after the boundary time, indicating efficient merger--driven fueling in the absence of long--lived bar--driven secular inflow.