The Milky Way may not have always looked the way we see it today. New research presented at the Royal Astronomical Society\u2019s National Astronomy Meeting suggests that our galaxy\u2019s disc may have undergone a dramatic flip in orientation during its history, rotating by more than 90 degrees.<\/p>\n
Astronomers at Durham University used supercomputer simulations of Milky Way-like galaxies<\/a> to investigate why our galaxy\u2019s stellar halo, the vast, faint cloud of stars surrounding the disc, rotates so slowly. The simulations revealed that galaxies with sluggish haloes often share two features: a major head-on merger with another galaxy and a subsequent disc flip.<\/p>\n Lead researcher Kirill Batrakov explained<\/a>: \u201cWe already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus. So, we think that the Milky Way disc likely flipped in the past.\u201d<\/p>\n That collision, about 10\u201311 billion years ago, was the largest in the Milky Way\u2019s early history<\/a>. It reshaped the galaxy and left billions of stars on elongated, sausage-shaped orbits. The new study suggests it may also have tipped the entire disc into a new orientation.<\/p>\n A disc flip means that most stars once moved on very different paths than they do today, including possibly our own Sun. It provides clues about the motion of the Milky Way\u2019s dark matter halo<\/a>, which appears closely linked to the stellar halo\u2019s rotation<\/a>. Also, it offers a new way to reconstruct the galaxy\u2019s complex past using present-day observations.<\/p>\n Batrakov noted: \u201cFinding that its disc flipped adds a new chapter to that story, one we must account for when placing the Milky Way in a broader context of other galaxies. What excites me the most is that this complex history can be reconstructed just from present-day observations.\u201d<\/p>\n