Riding the Galaxy’s Carousel: Measuring the Milky Way’s Rotation with Gaia Cepheids

Using nearly a thousand Classical Cepheids from Gaia DR3, Feng and collaborators construct a new and highly precise rotation curve of the Milky Way. Because Cepheids have well-defined rhythms of brightness that reveal their distances with exceptional accuracy, they are ideal tracers for mapping how fast different parts of our Galaxy spin. The authors begin by explaining why the rotation curve, which shows how orbital speed changes with distance from the Galactic center, is a powerful tool for understanding the distribution of mass, including dark matter. They then describe how Gaia’s improved measurements help overcome long-standing difficulties in gathering full three-dimensional velocities for stars spread across the disk.

Sample Selection and Distance Measurements

To build their sample, the team selects stars classified as Classical Cepheids in Gaia DR3 and further restricts them using quality requirements on proper motions and radial velocities. They apply cuts in sky position to avoid contamination from other galaxies and remove stars whose motions indicate they may not belong to the thin Galactic disk. The resulting sample of 903 Cepheids occupies a wide radial range and is concentrated near the Galactic plane. With accurate distances derived through the Period Wesenheit relation, the authors compute full positions and velocities for each star.

Asymmetric Drift and Velocity Analysis

Before constructing the rotation curve, the authors examine a subtle effect known as asymmetric drift, which causes the average azimuthal velocity of a stellar population to fall slightly below the true circular speed. While this effect is usually important for older and dynamically hotter stars, Cepheids are young and kinematically cold. The smaller radial velocity dispersions in their sample demonstrate this clearly. Using the Jeans equation framework, Feng et al. compute the asymmetric drift and confirm that it contributes only about 1.6 km per second, which is small but still included to ensure accuracy.

The Milky Way’s Rotation Curve

With these considerations in place, the authors derive the Milky Way’s rotation curve. The curve declines gently from about 242 kilometers per second at roughly 6.6 kiloparsecs to about 224 kilometers per second at roughly 17.6 kiloparsecs. Superimposed on this global trend is a notable dip around 10 to 11 kiloparsecs followed by a bump near 13 to 14 kiloparsecs. Similar structures have appeared in several previous studies using young tracers. Feng and collaborators discuss a variety of proposed explanations, including perturbations from the Galactic bar, spiral arm resonances, and features linked to ridge lines seen in the velocity radius plane, but emphasize that the phenomenon remains unsettled.

Averaged Rotation Curve and Mass Modeling

To avoid biasing their mass modeling with this dip and bump structure, the authors construct an averaged rotation curve using their own measurements together with several recent studies, restricting the radial range to parts of the disk less affected by fluctuations. From this averaged curve they obtain a circular speed at the Sun’s radius of 236.8 plus or minus 0.8 kilometers per second, agreeing well with other modern measurements. The authors then fit several Milky Way mass models, each combining different assumptions about the Galaxy’s baryonic components, with a standard Navarro Frenk White dark matter halo. Across all models, the resulting local dark matter density lies between 0.33 and 0.40 GeV per cubic centimeter, and the enclosed dark matter mass within 18 kiloparsecs ranges from 1.19 to 1.45 times ten to the eleven solar masses, values consistent with earlier rotation curve studies.

Conclusion

The paper closes by noting that while the rotation curve remains a powerful tool for probing the Galaxy’s mass distribution, it offers only part of the full picture. Future work, including better modeling of the outer disk, improved distance measurements, and complementary constraints from tidal streams and globular clusters, will be essential for sharpening our understanding of the Milky Way’s dark matter halo.

Source: Feng