How the Shape of a Galaxy Shapes Its Stars

Introduction

Galaxies are shaped not only by the stars they contain, but also by the invisible gravitational structures that guide their motion. In a new study, astronomers introduce the Rhea simulations, a set of detailed numerical experiments designed to explore how the gravitational potential of a Milky-Way-like galaxy influences its appearance and the way stars form within it. Rather than focusing on many different galaxies, the authors concentrate on one familiar type: a spiral galaxy similar in mass and structure to our own.

The central question of the study is simple but fundamental. If two galaxies have the same overall mass and gas content, but differ in how their gravitational potential is modeled, will they evolve in the same way? And if not, where do the differences appear?

To answer this, the researchers ran two large-scale simulations that are identical in almost every respect, except for how the galaxy’s gravitational potential is represented. By comparing the results, they were able to isolate the effects of specific structural components, such as spiral arms and the central bar, on the galaxy’s morphology and star formation.

The Rhea simulations are designed to capture the complex behavior of gas, stars, and feedback processes in a self-consistent way. They follow the evolution of a disk galaxy over several billion years, allowing the authors to examine both global trends and localized effects within different regions of the galaxy.

Two Ways to Model a Galaxy’s Gravity

In the first simulation, the gravitational potential is constructed to closely match a wide range of observational constraints for the Milky Way. This includes contributions from the disk, bulge, dark matter halo, spiral arms, and a central bar. The goal is to create a realistic gravitational environment that resembles what astronomers infer from observations of our own galaxy.

The second simulation uses a simpler approach. While it still represents a Milky-Way-like system with similar mass and rotation properties, it omits some of the more detailed non-axisymmetric structures. In particular, the gravitational influence of spiral arms and the bar is treated differently, resulting in a smoother and more idealized potential.

Aside from this difference, all other aspects of the simulations are kept the same. Both runs include the same prescriptions for gas cooling, star formation, and stellar feedback. This careful setup ensures that any differences that emerge can be attributed directly to the way the gravitational potential is modeled.

Similar Galaxies on Large Scales

When the authors compared the two simulations, one of the first results was how similar the galaxies looked on large scales. In both cases, the simulated galaxies developed extended disks with spiral patterns and overall morphologies characteristic of Milky-Way-like systems.

The global properties of the galaxies, such as their total star formation rates, were nearly identical. Over the course of the simulations, both galaxies formed stars at comparable rates, despite the differences in their underlying gravitational potentials. This suggests that, at least at the level of the entire galaxy, star formation is relatively insensitive to the specific details of how the gravitational field is modeled.

This result is important because it shows that simplified gravitational models can still reproduce many of the broad features observed in real spiral galaxies. For studies focused on global averages or long-term trends, a highly detailed potential may not always be necessary.

Differences in the Inner Regions

While the galaxies appear similar overall, the authors found clear differences when they examined the inner regions more closely. These differences are most pronounced near the center of the galaxy, where the influence of the bar becomes important.

In the simulation that includes a detailed bar potential, gas flows are more strongly affected in the central few kiloparsecs. The bar drives gas toward the center, altering its distribution and dynamics. This, in turn, influences where and when stars form in this region.

By contrast, in the simpler simulation, the absence of a comparable bar structure leads to a different pattern of gas motion in the inner disk. As a result, the morphology of the central region differs between the two runs, even though the outer parts of the galaxies remain broadly similar.

These findings highlight that while global properties may be robust, local structures can be highly sensitive to the details of the gravitational potential.

Star Formation in Groups

The authors also examined how stars form in groups within the simulated galaxies. They identified stellar groups and analyzed their properties, such as size and formation time, in different regions of the disk.

They found that the inclusion of a spiral arm potential has little effect on the properties of these stellar groups. Whether or not the spiral arms are modeled in detail, the resulting groups show similar characteristics in terms of their formation and evolution.

The situation is different for the bar. In the inner regions of the galaxy, particularly within about 2.5 kiloparsecs of the center, the bar potential has a noticeable impact. Stellar groups formed in this region tend to differ in size and formation times compared to those formed in the simulation without a detailed bar.

This result reinforces the idea that the bar plays a key role in shaping the inner galaxy, influencing not just the distribution of gas but also the way stars assemble into groups.

What the Rhea Simulations Show

Taken together, the Rhea simulations demonstrate that the gravitational potential of a galaxy matters in nuanced ways. On large scales, different modeling choices can lead to remarkably similar outcomes. But on smaller scales, especially in the central regions, the details become important.

The study shows that including a realistic bar potential can change the morphology and star formation behavior in the inner disk, while the spiral arm potential has a more limited effect on the properties of stellar groups. At the same time, the overall star formation rate of the galaxy remains largely unchanged.

By isolating these effects in a controlled set of simulations, the authors provide a clearer picture of which aspects of galactic structure are most sensitive to gravitational modeling.

Conclusion

The Rhea simulations offer a focused look at how the structure of a galaxy’s gravitational potential influences its evolution. The results suggest that simplified models can capture many global features of Milky-Way-like galaxies, but that detailed structures such as the central bar are essential for understanding the inner regions.

Rather than overturning existing ideas, the study adds clarity to where complexity matters most. For astronomers building models of galaxies, it provides guidance on when detailed gravitational components are necessary and when simpler approaches may suffice.

As future studies build on this work, the Rhea simulations serve as a reference point for exploring how galaxies like our own take shape over cosmic time.

Original source:
Introducing the Rhea simulations of Milky-Way-like galaxies I: Effect of gravitational potential on morphology and star formation, Astronomy & Astrophysics.
https://www.aanda.org/articles/aa/full_html/2025/12/aa52223-24/aa52223-24.html