Spinning Stars and Galactic Clues: How Stellar Motions Reveal the History of Our Galaxy's Bulge
In their study Luis M. San Martin Fernandez and collaborators investigate how stars move within the central region—or bulge—of a galaxy similar to the Milky Way. They focus on the shape and orientation of velocity ellipsoids, which describe how stars spread out in different directions of motion. A central quantity in this analysis is the vertex deviation, a measure of how much the dominant direction of motion tilts away from expected axes in a symmetric galaxy. This tilt can reveal the influence of the galactic bar—a long, dense structure of stars running through the bulge—on stellar orbits.
Simulating a Barred Galaxy
To explore these ideas, the team used a high-resolution simulation that allows a galaxy to form and evolve in isolation over ten billion years. This simulated galaxy forms a bar and develops a box/peanut-shaped (BP) bulge, similar to what is observed in the Milky Way. The stars that form in this galaxy are grouped by age: "young" stars between 4–7 billion years old, and "old" stars about 10 billion years old. These groups are not necessarily young in an everyday sense, but relatively young compared to the oldest populations in the bulge. The simulation allows the authors to observe how each population responds to the bar over time.
How Different Populations Respond to the Bar
The young stars form a strong, well-defined bar and display prominent X-shaped structures in the bulge when viewed side-on. These stars show strong streaming motions along the bar, which results in tilted velocity ellipses and a high vertex deviation. In contrast, the old stars form a more spherical, boxy bulge and are less affected by the bar. Their motion is more uniform and aligned with the galactic center, leading to smaller vertex deviations. This difference in motion—known as kinematic fractionation—is a key result: different stellar populations "feel" the bar differently based on their age and initial motions.
From the Galaxy’s Center to the Sun’s View
To better compare with observations, the authors transform their simulated data from a galactocentric (center-focused) frame to a heliocentric (Sun-centered) one. This step is necessary because most real measurements are taken from Earth. They find that the clear patterns seen in the simulation—especially the strong vertex deviation in young stars—are still present, though slightly distorted by perspective effects. These distortions reflect how projection into the sky affects our view of stellar motions, but they do not erase the essential differences between the young and old populations.
Comparing Simulation and Reality
Using data from two major astronomical surveys—APOGEE and Gaia—the researchers split observed stars into groups based on metallicity, or the abundance of heavy elements. Metal-rich stars are generally younger, while metal-poor stars are older. When the team compares these real stellar motions to their simulation, they find good agreement. The metal-rich stars in the data show strong vertex deviations and correlations, while the metal-poor stars do not—matching the behavior of young and old stars in the model. This supports the idea that observed kinematic differences are rooted in the bar's influence rather than requiring separate galaxy components.
A Bulge-Wide View of Kinematics
The study then zooms out to look at how the velocity ellipsoids vary across the entire bulge. They map anisotropy (uneven spread of motion), correlation (how two directions of motion relate), and vertex deviation throughout the bulge region. Again, the simulation matches the observations well: younger stars show complex structures shaped by the bar, while older stars remain more uniform. These results provide strong, visual evidence of how the bar sculpts stellar motions differently depending on age and kinematic history.
Conclusions: A Unified Origin for the Bulge
This research suggests that many features of the Milky Way’s bulge—such as its shape, stellar motions, and metallicity trends—can be explained by internal evolution alone. The bar plays a central role in separating stellar populations over time, a process known as secular evolution. The authors argue that these trends do not require the existence of a massive "classical" bulge formed by galaxy mergers. Instead, the Milky Way’s bulge may have developed primarily through kinematic fractionation, where different stars respond differently to the bar depending on their age and motion, offering a unified and elegant explanation of its complex structure.
Source: San Martin Fernandez
