Unwinding the Mystery of the Phase Spiral in the Milky Way

The vertical motion of stars in the Milky Way disk doesn’t follow the smooth, ordered paths that scientists once expected. Instead, stars trace out a striking spiral pattern in the space of vertical position and velocity, called the “phase spiral.” First discovered in 2018 with data from the European Space Agency's Gaia satellite, this pattern hints at a dramatic event in the Galaxy’s past, like a galactic collision or internal upheaval. In a new study, Axel Widmark and collaborators dive deeper into the shape and structure of the phase spiral across the Milky Way disk, searching for clues about how it formed and what it can reveal about the Galaxy itself.

Theories and Tools

To begin, the authors outline how the phase spiral can vary based on a star’s location and motion through the Galaxy. This spiral appears in a plot of a star’s vertical height above the disk versus its vertical speed, and it results from a past perturbation, something that knocked stars out of their vertical equilibrium. Scientists believe the spiral might have been caused by the gravitational tug of the Sagittarius dwarf galaxy, which passed through the Milky Way in the past, or by internal structures like the bar and spiral arms.

Mapping the Milky Way in Motion

To trace the spiral’s shape and evolution, the team uses precise measurements from Gaia’s third data release. They divide the Galaxy into a grid, collecting and analyzing star samples based on their positions (out to 4,000 parsecs from the Sun) and their orbital motions in the disk. Because direct measurements of stellar motion along the line of sight are not always available, the authors supplement the data with predictions from a machine learning model, trained to estimate these velocities with high accuracy.

Spiral Structure in Detail

The key spiral features they focus on are its “winding” (how tightly it coils), its “rotation phase” (the spiral’s orientation in the plot), and its amplitude (how strong the pattern appears). They also study the underlying gravitational pull that affects stars’ vertical motion, which depends on the mass density of the disk. Interestingly, they find that the strength of the gravitational pull decreases with distance from the Galactic center, matching expectations for a disk that becomes less dense farther out.

A Uniform Twist Across the Galaxy

One of the study’s major findings is that while the winding and amplitude of the phase spiral change with position and motion, the rotation phase is surprisingly smooth and coherent across the entire disk. This uniformity suggests that the phase spiral was not created by many small, random events, but rather by a large, galaxy-wide disturbance. This is important because it helps rule out some possible formation theories that predict more chaotic behavior in the spiral’s orientation.

A Curious Gradient in Time

Another key result is the strong variation in “winding time”, a way to estimate how long the spiral has been evolving, across different parts of the Galaxy. The inner regions of the disk appear to have wound more tightly than the outer ones, which suggests that the inner spiral has been winding for a longer time, or that it wound more quickly due to the higher mass density there. However, this conflicts with the idea of a single event affecting the entire disk at once, presenting a puzzle that future models and simulations must solve.

What Comes Next

The team compares their observations with previous studies and simulations, finding overall agreement with the idea that the phase spiral is linked to a global, rather than local, event. But some features, like how winding relates to the Galaxy’s spiral arms, are still not fully explained. These results set the stage for future investigations that could not only pinpoint the source of the spiral, but also provide new insight into the shape and mass of the Milky Way disk.

Conclusion

Widmark and colleagues show that the phase spiral is a powerful tool for understanding the recent history and structure of our Galaxy. Their detailed maps and careful analysis bring us one step closer to untangling the Milky Way’s dynamic past.

Source: Widmark