The Milky Way May Have Changed Orientation After an Ancient Galactic Collision

The Milky Way may have changed its orientation after an ancient collision with another galaxy. This is the possibility arising from simulations conducted with supercomputers by researchers at Durham University in the UK, whose results were presented at the national astronomy conference of the Royal Astronomical Society in Birmingham.

The study analyzed galaxies similar to ours to try to explain a characteristic observed in the Milky Way: the very slow rotation of its stellar halo. This halo surrounds the spiral disk where most of the stars in the galaxy are located and is largely composed of stars that formed in smaller galaxies that were later absorbed through galactic mergers.

Observations from the Gaia mission of the European Space Agency (ESA) had shown that the stellar halo of the Milky Way rotates at a very low speed. To understand this behavior, researchers studied the evolution of 25 similar galaxies using the Auriga cosmological simulation set.

The analysis followed the evolution of these galaxies over billions of years. The researchers found that galaxies with slower rotating stellar halos had undergone a significant head-on merger with another galaxy.

During this process, their disks changed orientation throughout their evolution. From this relationship, the team proposed that something similar may have occurred with the Milky Way.

"We already know that the Milky Way had a massive head-on collision with a galaxy known as the Sausage or Gaia-Enceladus. For this reason, we believe that the disk of the Milky Way likely flipped in the past," explained astronomer Kirill Batrakov.

Gaia-Enceladus was a massive dwarf galaxy that collided with the primordial Milky Way about 10 to 11 billion years ago. It was then absorbed by our galaxy as part of a merger that, according to the study, was the largest event in the early history of the Milky Way.

This collision transformed the galactic structure and left billions of stars orbiting in very elongated trajectories. This movement pattern was named "Sausage" due to its characteristic shape.

The new study links this episode to the orientation of the galactic disk and the slow rotation of the stellar halo. According to the simulations, a significant head-on merger can alter the subsequent evolution of a galaxy and modify the position of its disk.

Batrakov's work also indicates that the rotation of the stellar halo of the Milky Way is linked to the rotation of its dark matter halo.

This relationship suggests that both components would have evolved together during the growth of the galaxy. The stellar halo, formed by stars captured from smaller galaxies, and the dark matter halo would be part of the same accumulation and merger story.

The hypothesis aims to reconstruct how the Milky Way took shape over billions of years. The simulations do not show a direct observation of the galactic past, but they allow for comparisons of the evolution of similar galaxies and assess what processes may explain the current structure.

Most of the stars in the Milky Way are found in its flat spiral disk. However, the stellar halo surrounding it retains signals of ancient mergers and absorptions of smaller galaxies.

Therefore, studying its movement provides information about events that occurred in the early stages of the galaxy. In this case, the slow rotation of the halo and the possible flipping of the disk appear to be associated with the ancient collision with Gaia-Enceladus.

The results of the simulations add a new clue about the formation of the Milky Way. According to the researchers, the galaxy we inhabit today may have changed its orientation as a consequence of one of the most significant impacts in its history.

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