Spinning Up the Galaxy: How the Milky Way’s Bar Transfers Motion to Its Bulge and Halo

Using data from Gaia’s third data release, which contains precise positions and motions for over a billion stars, Zhuohan Li and collaborators sought to better understand an unexpected rotational signal in the inner Milky Way. They applied a neural network (NN) model trained to differentiate between stars formed within the Milky Way (in-situ) and those that originated elsewhere (ex-situ). Among the 27 million stars examined, the model highlighted a distinct group of about 1.2 million stars with unusual kinematics. These stars were rotating faster than expected for halo or bulge stars, but not as quickly as the thin or thick disk, suggesting a unique dynamical history worth exploring further.

Modeling a Decelerating Bar

To investigate the source of this unexpected rotation, the authors turned to simulations of the Galaxy that included a decelerating central bar—an elongated structure of stars near the Milky Way’s center. As the bar slows down due to a process known as dynamical friction, it can transfer angular momentum to other stars. The authors used the AGAMA code to simulate the movement of test particles in this evolving gravitational environment, incorporating known parameters for the Milky Way’s mass distribution and bar pattern speed. This allowed them to track how the bar’s slow-down could influence the motion of surrounding stars over billions of years.

Breaking Down the Contributors

Analysis of the simulation showed that the unusual rotating population is a blend of bulge, halo, and high-α thick disk stars, with the bulge and halo being the main contributors. Spatially, the bulge stars tend to be located within 5 kiloparsecs of the Galactic center, while the halo stars dominate at larger distances, particularly near and beyond the Sun’s orbit. Importantly, the model revealed that these stars had gained angular momentum over time, explaining their net rotation. This mix of components aligns closely with what was observed in the Gaia data, lending strong support to the idea that the decelerating bar is driving the observed motion.

How Angular Momentum is Passed On

The process by which the bar transfers spin to surrounding stars relies heavily on resonance trapping, where stars interact with the bar’s pattern at specific orbital frequencies. Bulge stars near the Galactic center and halo stars at greater distances both showed evidence of gaining angular momentum through this mechanism, particularly around the bar’s corotation resonance. However, not all the rotation came from this specific interaction—some stars appeared to have gained momentum through other, more subtle mechanisms. These findings point to a complex, ongoing exchange of angular momentum in the inner Galaxy, driven largely by the bar’s evolving dynamics.

A Galaxy in Motion

Ultimately, the study presents compelling evidence that both the Milky Way’s bulge and inner halo are rotating as a result of angular momentum transferred from the decelerating central bar. This marks the first time such rotation has been clearly identified in a million-star sample. The simulation predicts that the bar has slowed down by about 37.5% over the past 4 billion years, growing in mass and influence during that time. While the current model does not include effects like disk self-gravity or spiral arms, it provides a strong first-order explanation for how a once-static Galactic core can be spun into motion over cosmic timescales.

Source: Li