The Role of Galactic Bulges in Shaping Stellar Bars and Box-Peanut Features
Galaxies come in many forms, and some disk galaxies exhibit central bars—elongated structures of stars that influence their overall evolution. Alongside these bars, vertical extensions called Boxy/Peanut X-Features (BPX) often appear, giving insight into the galaxy's history. Rachel McClure and her team explore how the presence of a classical bulge—an older, spherical mass of stars—affects the development of bars and BPX structures through detailed simulations.
Galactic Bars and Bulges: The Basics
Galactic bars, observed in over 60% of nearby disk galaxies, emerge from the collective motion of stars. BPX structures, on the other hand, are vertically extended regions that appear boxy or X-shaped when viewed edge-on. These features are linked to the galaxy's bar, forming through the resonant interactions of stars with the bar's gravitational influence.
Classical bulges differ from pseudobulges. Classical bulges are formed by early galaxy mergers and are dominated by random star motions, while pseudobulges form gradually due to rotational forces. McClure’s study focuses on classical bulges to understand their role in the growth and stability of galactic bars and BPX features.
The Simulations
To study these dynamics, McClure's team used advanced computer simulations of disk galaxies with varying bulge mass fractions (0% to 16% of the disk's mass). These simulated galaxies were modeled to include three main components: a dark matter halo, a stellar disk, and a central bulge. The team analyzed the stellar orbits and tracked how the bars and BPX features evolved over billions of years.
How Bulges Influence Bars and BPX Growth
The simulations revealed that bulge mass strongly affects the bar’s strength and the rate of BPX formation. Heavier bulges stabilize the fraction of vertically extended orbits, preventing sudden changes in the galaxy's structure. Bars in galaxies with larger bulges grew longer and stronger but evolved more slowly. Conversely, galaxies without bulges have experienced more rapid, less stable changes.
BPX features emerged in all simulations but grew at different rates. In galaxies without bulges, the BPX formed quickly, often causing brief disruptions in the bar’s evolution. In galaxies with heavier bulges, the growth of the BPX was steadier, suggesting that the bulge helps moderate the process.
Resonances and Orbital Dynamics
The study showed that BPX features form when stars move through specific resonances—regions where the gravitational pull of the bar alters their orbits. Horizontal resonances align stars with the bar, while vertical resonances extend their orbits above and below the disk plane. The overlapping of these resonances is key to BPX growth. The simulations demonstrated that resonance interactions are sufficient to create BPX structures, even without sudden gravitational instabilities.
Conclusions
McClure and her team concluded that while classical bulges are not essential for BPX formation, they play a significant role in shaping how bars and BPX structures evolve. Heavier bulges lead to more stable and gradual changes, while lighter or absent bulges result in more dynamic and rapid transformations.
Source: McClure
