A possible answer to a long-standing Milky Way mystery
The Milky Way may have experienced a dramatic reorientation in its distant past, with its disk flipping by more than 90 degrees after a major collision. That is the central claim of new work presented by astronomers from Durham University, who used supercomputer simulations to investigate why the stellar halo around our galaxy rotates so slowly.
The idea is striking because the Milky Way is often imagined as a stable spiral system whose broad structure has remained fixed over cosmic time. The new research argues that the galaxy’s history may have been far more dynamic. If the findings hold up, they would tie a puzzling feature seen in modern observations to a violent event during the Milky Way’s early assembly, when mergers with smaller galaxies helped build the system we see today.
The study was presented at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham. Rather than relying only on a single reconstructed history for our galaxy, the team examined a larger set of simulated Milky Way-like systems to identify patterns that could explain the behavior of the Milky Way’s halo.
The clue lies beyond the spiral disk
Most stars in the Milky Way are concentrated in the familiar flat spiral disk, where the Sun also resides. But the galaxy is surrounded by a much larger and far more diffuse stellar halo. This halo is populated largely by stars that did not originally form in the main body of the Milky Way. Instead, many of them were born in smaller galaxies and later pulled into the system through mergers.
That makes the halo a kind of fossil record of galactic assembly. Unlike the bright disk, which is dominated by ongoing structure and motion shaped by the galaxy’s current gravitational configuration, the halo preserves evidence of earlier disruptions. Recent observations from the European Space Agency’s Gaia mission showed that the Milky Way’s stellar halo rotates only very slowly. Astronomers have known this, but the reason has remained unclear.
The Durham team set out to test whether that slow rotation could be a signature of a specific kind of past event. They analyzed the histories of 25 Milky Way-like galaxies in the Auriga suite of cosmological simulations, following how those systems developed across billions of years.
A pattern emerges in the simulations
According to the researchers, the galaxies with the slowest-rotating stellar halos tended to share two traits. First, they had undergone a major head-on merger with another galaxy. Second, they had experienced what the researchers call a disk flip, meaning the orientation of the galaxy’s disk changed by more than 90 degrees over time.
That pairing is important. A major merger can redistribute stars, alter angular momentum and disrupt preexisting structure. But the work suggests that the most revealing systems are those where the collision was not just large, but transformative enough to change the disk’s orientation. In that scenario, the stars captured into the halo from earlier interactions and infall events would end up carrying motion that no longer aligns cleanly with the final disk. The result could be a halo with much weaker overall rotation.
The research therefore offers a possible explanation for the Milky Way’s present-day halo: the galaxy may have suffered a direct and consequential encounter during its early formation, then settled into a new orientation while retaining a halo imprinted by the disruption.
Connecting the theory to Gaia-Enceladus
The source material places this idea in the broader context of the Gaia-Enceladus merger, a major event that is already thought to have occurred roughly 10 billion years ago during the Milky Way’s early formation stages. That merger has become one of the best-known chapters in the story of how the Milky Way grew, because Gaia data revealed a large population of stars whose motions point to an ancient absorbed galaxy.
The new interpretation does not simply repeat that the Milky Way merged with another galaxy. Instead, it raises the possibility that such a merger could have done more than add stars to the halo and thicken the galaxy’s structure. It may have changed the orientation of the entire disk. That is a much more dramatic claim, and one that would reshape how the public imagines the Milky Way’s evolution.
If correct, the work would show that a galaxy can preserve evidence of an extreme internal reordering for billions of years, not in a single obvious visible scar, but in the subtle collective motion of stars far from the bright central disk.
Why this matters beyond our own galaxy
The study is not just about the Milky Way. It also points to a broader method for reading galactic histories. If slowly rotating stellar halos are reliable indicators of past head-on mergers and disk flips, astronomers could use halo motion as a diagnostic tool for other spiral galaxies. Instead of treating halos merely as diffuse outskirts, they could become one of the most revealing components for reconstructing how galaxies assembled and how severely they were reshaped along the way.
That would be especially valuable because direct evidence of ancient mergers can fade or become hard to interpret over cosmic timescales. A disk may look orderly in the present day while still carrying a violent history in its outer stellar populations. The new work argues that those populations retain measurable signatures of upheaval.
There is also a conceptual payoff. Popular descriptions of galaxies often emphasize graceful long-lived structure: elegant spirals, stable rotations, predictable orbital patterns. But galaxy formation is, in reality, a process of accumulation, disruption and repeated gravitational negotiation. The Milky Way may appear calm from our vantage point inside it, yet its current form could be the outcome of a profound cosmic reorientation.
An early result with a provocative implication
The findings are still at the conference-presentation stage in the source material, so they should be read as a developing scientific result rather than the final word. Even so, the proposed explanation is notable because it directly links an observed present-day mystery to a testable evolutionary pathway seen in high-resolution simulations.
That makes the study more than a speculative narrative. It is a hypothesis grounded in a comparison across multiple simulated galaxies that resemble the Milky Way in broad terms. Future work will determine how tightly that pattern holds and how well it fits the full range of observational constraints from Gaia and other surveys.
For now, the research offers a vivid new possibility: the Milky Way did not simply grow by swallowing smaller galaxies. At least once, it may have been knocked so hard that the orientation of its disk flipped, leaving behind a stellar halo whose slow spin still records the aftermath today.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
