Tracing the Galactic Past: How Precession Shapes the Milky Way’s Stellar Streams
Stellar streams, ribbons of stars stretched out across the sky, are remnants of disrupted star clusters and dwarf galaxies that orbit the Milky Way (MW). Their paths through space hold clues about the Galaxy’s formation history. Elena Asencio and collaborators revisited whether these streams share a common origin with other MW structures, such as the “disc of satellites” (DoS) and young halo globular clusters, by studying the direction of their orbital poles. This direction, perpendicular to a stream’s orbital plane, can reveal whether multiple structures orbit in a coordinated way, as expected if they formed together in an event like a past encounter between the MW and the Andromeda galaxy (M31).
Data and Methods
The team used the most up-to-date galstreams catalogue, which contains 95 MW streams with detailed positional, distance, and motion data. After consolidating related streams into single entries, they analyzed 91 unique streams, focusing on those within 100 kiloparsecs (kpc) of the Galactic center. The authors compared the streams’ orbital poles to the orientation of the Vast Polar Structure (VPOS), a hypothesized plane in which many MW satellites orbit. While earlier studies hinted at strong alignment between stream poles and the VPOS, the expanded dataset shows no significant clustering for the full sample, suggesting the poles appear randomly oriented. However, streams farther from the Galactic center showed a stronger tendency to align with the VPOS.
The Role of Precession
To understand this trend, the authors explored the role of precession, a gradual tilting of orbits caused by the MW’s non-spherical gravitational field. Precession rates are faster for objects closer to the center, meaning inner streams have likely drifted far from their original orbital orientations over billions of years. Using a formula for precession in slightly flattened potentials, they estimated that streams at 20 kpc could have shifted by tens of degrees in just a few gigayears. This effect could erase any original alignment in the inner Galaxy while leaving the orbits of more distant streams relatively intact.
Comparison to Simulations
The team then compared their findings to a Milgromian dynamics (MOND) simulation of a past MW-M31 fly-by, which naturally produces tidal debris with an initially anisotropic pole distribution. In the simulation, debris at large distances (beyond ~150 kpc) still shows strong clustering today, while closer material appears more dispersed, matching the pattern seen in real data. Statistical tests on mock catalogues from the simulation confirmed that a strong preference for the VPOS should only emerge if future surveys can include streams at distances greater than ~150 kpc.
Reversing Precession Effects
When the authors “back-traced” the present-day poles of real streams by reversing the effects of precession over 6.6 Gyr, they found a more clustered distribution, consistent with the idea of an originally anisotropic system. Removing streams with very large uncertainties made this clustering even stronger. Additional hints came from a noticeable lack of streams on near-polar orbits, where precession-driven widening could destroy streams over time. The few polar streams observed are close to either the VPOS or the Sagittarius dwarf galaxy, suggesting a link to recent formation events.
Conclusions and Future Outlook
In conclusion, while the current dataset cannot definitively confirm that most MW streams belong to the VPOS, the observed distance-dependent alignment, the simulation comparisons, and the precession modeling all support the possibility of a common origin. The authors suggest that identifying and measuring more distant streams, beyond 150 kpc, could provide the decisive evidence. If confirmed, the MW-M31 fly-by scenario within a MOND framework would offer a compelling explanation for the VPOS and other large-scale structures in the MW.
Source: Asencio
