Galactic Ripples in a Turbulent Sea: Can Phase Spirals Survive the Clumpy Interstellar Medium?
In 2018, the Gaia satellite from the European Space Agency revealed an unexpected pattern in the Milky Way: a spiral shape in the “phase space” of stars near the Sun. When astronomers plotted each star’s height above the Galactic plane (z) against its vertical speed (Vz), they noticed a distinctive spiral. This feature—now called the “phase spiral”—is believed to be the Milky Way’s response to a past disturbance, possibly caused by the Sagittarius (Sgr) dwarf galaxy plunging through the Galactic disc hundreds of millions of years ago. The pattern is thought to result from “phase mixing,” a process where stars gradually settle after being disturbed.
Simulating a Realistic Galaxy
To understand whether the phase spiral can persist in more realistic conditions, Tepper-García and collaborators created detailed computer models of a Milky Way-like galaxy. They used a range of setups: some included only stars (called “pure N-body” simulations), others added gas that behaves in different ways—either staying cool and passive or actively forming stars and driving turbulence. Some models simulated the passage of a massive perturber like Sgr, while others let the galaxy evolve in isolation. This variety allowed them to test how gas properties and external influences affect the survival of the spiral pattern.
Gas Can Suppress or Support the Spiral
The simulations showed that the phase spiral forms clearly in a galaxy made only of stars, especially after an external disturbance like Sgr passes through. However, when even a modest amount of inert (non-star-forming) gas was added, the spiral became weaker and faded more quickly. In contrast, when the gas was allowed to cool, form stars, and become turbulent—more like real galactic gas—the phase spiral still appeared, although it was patchier and less long-lived. This suggests that feedback-driven turbulence can help sustain the spiral in a messy environment, even if it’s not as strong as in a clean, gas-free disc.
Phase Spirals Without an External Kick?
Interestingly, the team found that even in galaxies without an external impact, weak phase spirals could form if the gas was turbulent and star-forming. This may be explained by the “Tremaine-Frankel-Bovy (TFB) effect,” where small, random gravitational nudges from clumps of gas gradually create spiral-like features in phase space. These spirals were less organized but still showed some of the same kinematic patterns. However, in completely isolated, gas-free models, phase spirals were almost entirely absent—suggesting that clumpy gas can sometimes act like a substitute for a satellite galaxy, at least on small scales.
How Clumpy Is Too Clumpy?
To dig deeper, the authors analyzed the structure of the gas using a technique called a power spectrum, which shows how much structure exists at different size scales. They discovered that when the gas was extremely clumpy on small scales (less than 1 kiloparsec), it actually suppressed the phase spiral. This was especially clear in simulations where the gas couldn’t form stars and instead collapsed into dense blobs. In contrast, more balanced turbulence—like that produced by feedback from star formation—helped keep the gas less extreme, which allowed the spiral to survive in pockets.
Tracking the Spiral’s Strength Over Time
The researchers also tracked where and when the spiral was visible in the simulated galaxy by dividing the galactic disc into 12 regions and checking each one over time. They used a clever technique that looked for spiral-like signatures using Fourier analysis, which breaks patterns down into wave-like components. The spiral’s presence peaked around 700–800 million years after the satellite’s impact, but then faded. In models with gas, the spiral’s visibility depended on the gas's structure—highlighting that both timing and local conditions matter.
Conclusion: A Fragile Signal in a Chaotic Galaxy
This study demonstrates that the phase spiral is a delicate but powerful probe of the Milky Way’s history and structure. It thrives in quiet, smooth stellar discs and can survive in chaotic, star-forming environments—so long as the gas isn’t too clumpy. Gas affects the spiral both by changing the gravitational potential and by jostling stars with small, random kicks. Overall, the survival of the phase spiral depends on a careful balance of forces, and future observations and simulations may use it to map out the hidden forces shaping our Galaxy today.
Source: Tepper-García
