Scientists have discovered that the Milky Way galaxy likely underwent a significant "disc flip" in its distant past, a dramatic reorientation of its stellar disk by more than 90 degrees. This cosmic event, which dragged the solar system along with it, is believed to have occurred around 10 to 11 billion years ago following a collision with a massive dwarf galaxy known as Gaia–Sausage–Enceladus.
This finding emerged from a study analyzing the simulated evolution of 25 Milky Way-like galaxies. Researchers found that galaxies with the slowest-rotating stellar halos, like the Milky Way's, had two common characteristics: a head-on collision with another galaxy and a subsequent major disc flip. The Milky Way possesses both a slow-moving stellar halo and a known ancient head-on collision, making a disc flip a probable event in its history.
Galactic Collision and Halo Rotation
The study, presented at the Royal Astronomical Society's National Astronomy Meeting, aimed to solve the puzzle of the Milky Way's unusually slow-rotating stellar halo. This halo, a sparse region surrounding the flat spiral disk, is composed of stars pulled from other galaxies over time. Until now, the reason for its slow rotation, with stars taking up to a billion years to orbit the galactic core, remained unclear.
The simulations revealed that a disc flip would reorient the galaxy's stars onto new trajectories. Lead author Dr. Kirill Batrakov of Durham University stated that this means the solar system's position might not have been as stable throughout its lifetime as previously assumed. The flip would have significantly altered the movement of most stars within the Milky Way.
The collision with Gaia-Sausage-Enceladus is known to have sent billions of stars into looping, sausage-shaped paths. The new research suggests this impact was also responsible for flipping the galaxy's disk. The researchers also noted a link between the stellar halo's rotation and the galaxy's dark matter halo, a critical but invisible component that provides gravitational stability.
Understanding the origin of the Milky Way's slow-moving stellar halo may offer insights into the nature of dark matter. The study suggests that the Milky Way's complex history, including this disc flip, can be reconstructed through present-day observations, aiding in the understanding of galaxy evolution across the universe.