Why Do the Sun and the Stars in Our Galaxy Move Like This?

Gaia, the high‑precision space astronomy mission, has gathered data on over 55 million stars within 3‑6 kpc of the Sun. Using a new statistical method based on the five astrometric parameters (position, parallax, radial velocity, and two tangential velocity components), scientists calculated the Sun’s local motion: U≈9.6 km/s, V≈16.3 km/s, W≈7.3 km/s. They also measured the galaxy’s rotation speed around the Sun, Vc≈234 km/s, and found that rotation slows linearly with height above the galactic plane, at about 33.5 km/s per kpc. To interpret these data, researchers built a Monte‑Carlo model that simulates the galaxy as an axisymmetric system. By optimizing six free parameters, the model reproduces the observed velocity distributions in three dimensions. This suggests that, despite the complexity of stellar dynamics, a simple description can explain the collective motion of stars in our region of the galactic disk. This analysis shows how Gaia observations can transform our understanding of the Milky Way’s structure and evolution, providing a quantitative foundation for future studies on dark matter and star formation.

The study relies on advanced statistical analysis of Gaia DR3 astrometric data, which include angular position, parallax, radial velocity, and tangential velocity. To handle the large star sample (~55 M), the researchers developed a multivariate regression algorithm that estimates collective motion parameters (U, V, W) and galactic rotation (Vc) as functions of radial distance (R) and height (Z). Their procedure accounted for systematic errors (e.g., parallax bias) and normalized velocity distributions using bootstrap methods. The employed Monte‑Carlo model is an axisymmetric approach that assumes an exponential stellar density profile and rotation dependent only on R. The six free parameters include the rotation amplitude, the slope of velocity with Z, radial velocity dispersion, tangential velocity dispersion, Vc at R⊙, and an offset term. Optimization was performed via maximum likelihood, comparing generated velocity distributions with observed ones. Results show that within the 3‑6 kpc region, galactic rotation decreases linearly with Z, with a coefficient of 33.5 km s⁻¹ kpc⁻¹. This suggests a dynamic pressure gradient or a non‑uniform dark matter distribution. The precision of U, V, W (±0.05–0.9 km s⁻¹) confirms the method’s robustness and paves the way for more detailed studies of the Milky Way’s internal structure, including spiral arm and bar effects.