Measuring Turbulence: Key Quantities Behind the Driving Parameter
To calculate b, the authors measure three turbulence-related quantities from the WNM: the density dispersion (how much the density fluctuates), the turbulent sonic Mach number (how fast the gas moves compared to the speed of sound), and the plasma beta (the ratio of thermal to magnetic pressure). After filtering out non-turbulent trends, they find typical turbulent Mach numbers of about 1–3, meaning the WNM flows are roughly transsonic to mildly supersonic. The plasma beta is usually between 3 and 13, implying that thermal pressure dominates over magnetic pressure in this phase of the ISM. These values fall within ranges expected for Milky Way environments.
Time Evolution: A Cycle of Compression, Star Formation, and Feedback
A major result of the study is that the turbulence driving parameter b varies significantly over time, ranging from about 0.4 to 1.0. This means the turbulence shifts between nearly natural mixtures of stirring motions and strongly compressive states. Importantly, Gerrard finds that periods of highly compressive driving (b > 0.5) tend to occur about 10 million years before peaks in star formation, which is roughly the turbulent turnover time on 100-pc scales. In other words, compressive turbulence first gathers gas and increases density, and only after some delay does star formation rise in response. This pattern repeats across the ∼100 Myr examined in the simulation.
Supernova Influence: How Feedback Reshapes the Turbulence
After a burst of star formation, the resulting supernova feedback creates a second, delayed effect: it increases the turbulent Mach number, boosts the amount of WNM gas, and lowers plasma beta. These supernova-driven shocks disturb the gas about 20 Myr after star formation peaks, reducing b back toward ∼0.4–0.5, the value associated with a natural mix of driving modes. Thus, the authors find a cyclical relationship: compressive turbulence → star formation → supernova feedback → reduced compressive driving.
Comparison with Observations: A Realistic Picture of the ISM
Finally, Gerrard compares the simulation results to real astronomical measurements of turbulence in the Milky Way and nearby galaxies. The values of b in the simulation match well with observational estimates, which also suggest that WNM turbulence is often more compressive than solenoidal. The study provides one of the first detailed looks at how b evolves naturally in a realistic galactic environment, demonstrating that the dominant turbulence-driving mode is both variable and closely linked to the timing of star formation and supernova feedback.
Conclusion: Turbulence as a Dynamic Driver of Star Formation
Overall, the paper shows that turbulence in galaxies is not a fixed background process but a dynamic, evolving driver of star formation. By examining how compressive and solenoidal motions rise and fall over tens of millions of years, Gerrard and Federrath highlight the importance of timing in the star formation cycle, and provide a framework for comparing simulations directly with observations in the future.
Source: Gerrard
