Is the Milky Way Really Slowing Down? A Closer Look at the Galaxy’s Rotation Curve
In recent years, a few papers have proposed that the Milky Way’s rotation curve, how fast stars orbit at different distances from the center, appears to decline at larger distances from the Galactic center, similar to how planets slow down the farther they are from the Sun. This so-called "Keplerian" decline is surprising, because other spiral galaxies typically show “flat” rotation curves, where stars orbit at nearly constant speeds, even far from the center. In their new paper, Klacka and Šturc revisit this issue and argue that the apparent decrease in rotation speed may not reflect how the Milky Way truly behaves.
The Problem with Using the Wrong Equations
The authors begin by pointing out a problem in how recent studies, such as those by Ou et al. (2024), Jiao et al. (2023), and Sylos Labini et al. (2023), analyzed their data. These papers used a mathematical tool called the axisymmetric Jeans equations, which are only valid when a galaxy’s mass is distributed evenly in a flat, disk-like shape. However, Klacka and Šturc argue that this assumption breaks down at large distances, beyond about 20 kiloparsecs (roughly 65,000 light-years), where the Milky Way’s matter is more spherically distributed. Using the wrong model can lead to incorrect conclusions about the galaxy’s rotation.
Questioning the Reported Velocity Drop
In the next section, the authors take a closer look at the rotation curve measurements published by Ou et al. (2024). They show that at a distance of 27.3 kpc from the Galactic center, the measured circular velocity is 173.0 ± 17.1 km/s, much lower than the 230 km/s expected near the Sun. This significant drop seems to support a Keplerian decline. But Klacka and Šturc argue that this drop is an artifact caused by applying the wrong equations.
Spherical Mass Dominance at Large Distances
To support their argument, the authors compare how much of the observed acceleration is due to the galaxy’s flat disk (like stars and gas) versus its spherical components (such as dark matter and the central bulge). At these large distances, over 80% of the gravitational acceleration comes from the spherical components. This means the Milky Way behaves more like a sphere than a disk out there, so the spherical version of the Jeans equations should be used instead.
Reinterpreting the Rotation Curve
By applying the correct spherical equations, Klacka and Šturc show that the rotation speed at 27.3 kpc should actually be closer to 245 km/s, which aligns well with a flat rotation curve. This result means the Milky Way might not be so different from other spiral galaxies after all. The apparent drop in speed in previous studies likely came from misapplying the disk-based Jeans equations, not from a real physical change.
Warped Structure Strengthens the Case
The paper also touches on another important factor: the warp in the Milky Way's disk. Observations show that the outer parts of the galaxy are bent or warped, especially for older stars. This further challenges the idea that the galaxy’s mass is evenly spread in a flat disk, reinforcing the need to use spherical models for stars far from the center.
Final Thoughts and Broader Implications
Klacka and Šturc argue that there is no real evidence of a Keplerian decline in the Milky Way’s rotation curve. Instead, the drop seen in some studies is likely due to using the wrong mathematical tools. When analyzed properly, the Milky Way appears to have a flat rotation curve, just like other spiral galaxies. This has major implications for how we estimate the amount of dark matter in our galaxy and how we compare it to others.
Source: Klacka
