How Do Galactic Bars Form? A Look at Tidal vs. Internal Growth
Galaxies, like the Milky Way, often have a central elongated structure known as a bar, which plays a crucial role in shaping their evolution. But how do these bars form? Some emerge due to internal instabilities within a galaxy, while others are externally triggered by the gravitational pull of passing galaxies. In this study, Yirui Zheng and colleagues explore whether bars that form due to external interactions grow faster than those that develop internally.
Simulating Galaxies and Bars
To compare these two types of bars, the authors used computer simulations that model how galaxies evolve over time. They tested three types of disk galaxies: a cold disk that is naturally unstable and forms bars easily, a warm disk that is marginally stable, and a hot disk that resists bar formation. By running simulations both in isolation and under the influence of a passing galaxy, the researchers could track how bars formed and evolved.
They also varied the size and trajectory of the passing galaxy, simulating different types of tidal interactions to see how external forces influenced bar formation. The key quantity they measured was the bar growth timescale, which describes how quickly a bar strengthens over time.
Do Bars Form Faster When Galaxies Interact?
The results showed that bars in cold and warm disks grew at similar rates, regardless of whether they formed spontaneously or were triggered by external forces. However, tidal interactions (such as a galaxy flyby) could either accelerate or delay the onset of bar formation. If the external force aligned with the natural tendency of the galaxy to form a bar, the bar appeared sooner than in isolation. If the tidal force opposed this tendency, it could temporarily delay bar formation.
Interestingly, hot disks—which wouldn’t form bars on their own—could develop bars under strong tidal forces. However, these bars didn’t grow in the same way. Instead of the exponential growth predicted by the standard swing amplification theory, the bars in hot disks grew more linearly, meaning their strength increased at a steady rate over time.
What This Means for Galaxy Evolution
This study suggests that a galaxy's internal structure is the dominant factor in determining how its bar grows. Tidal forces don’t necessarily make bars grow faster; they mainly influence when a bar starts forming. For galaxies already prone to forming bars, interactions just change the timing, not the speed. However, in galaxies that are resistant to bar formation, tidal forces can still induce bars, but these bars develop in a different way.
Source: Zheng
