Astronomers now believe a dramatic cosmic collision 10 billion years ago may have completely reoriented the Milky Way galaxy. Researchers say a direct hit from a dwarf galaxy, known as the Gaia Sausage, likely flipped our galaxy's entire disc by more than 90 degrees into its current position. This massive event happened long before our solar system even formed.[theguardian+1]
The Ancient Cosmic Crash
The cataclysmic collision occurred when the Milky Way was much younger. The incoming Gaia Sausage dwarf galaxy slammed head-on into our galaxy. While the Milky Way ultimately tore the smaller galaxy apart and absorbed its stars, the impact was powerful enough to slowly rotate the Milky Way's disc.This process, called a "disc flip," gradually reoriented the galaxy over hundreds of millions of years.[theguardian+3]
The Gaia Sausage, also sometimes called Gaia-Sausage-Enceladus, was a significant dwarf galaxy. It contained stars, gas, and dark matter with a combined mass exceeding 10 billion times that of our Sun.This ancient merger was a defining event in the early history of the Milky Way, profoundly reshaping its structure.[timesofindia+2]
Kirill Batrakov, the lead researcher on the project from Durham University, explained that the transformation was not instantaneous. "It probably takes at least a few hundred million years," Batrakov said.The findings were recently presented at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.[theguardian+3]
Solving a Galactic Mystery
The research began as astronomers tried to solve a long-standing puzzle about the movement of stars in the Milky Way's stellar halo. The stellar halo is a vast, spherical cloud of stars surrounding the galaxy's main disc.Observations from the European Space Agency's Gaia mission showed that stars in the Milky Way's disc orbit the galactic center at about 220 kilometers per second.However, stars in the halo move much more slowly, rotating at only about 25 kilometers per second.This significant difference puzzled scientists for years.[theguardian+6]
To understand this disparity, the Durham team used advanced supercomputer simulations. They modeled the evolution of Milky Way-like galaxies over billions of years. The simulations revealed that galaxies with unusually slow-rotating stellar haloes often shared two key characteristics: they had experienced a major head-on merger with another galaxy and had also undergone a disc flip during their history.[sciencesources+1]
"We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus," Batrakov told ZME Science. "So, we think that the Milky Way disc likely flipped in the past."The peculiar, elongated "sausage-shaped" orbits of stars in the halo were a key clue, tracing back to the dwarf galaxy breaking up as it plunged into the Milky Way.[zmescience+1]
Reshaping Our Galaxy
This ancient collision fundamentally reshaped the Milky Way. It played a crucial role in creating the central bulge of our galaxy and dominating much of its stellar halo.The interaction scattered debris from the Gaia Sausage throughout the inner parts of the Milky Way.[simonsfoundation+2]
Identifying this past disc flip offers astronomers a new way to understand how the Milky Way assembled over cosmic time. It may also provide indirect clues about the motion of its invisible dark matter halo.This research highlights the violent and dynamic history of galaxies, showing how major mergers can leave spiral galaxies looking surprisingly normal today, despite dramatic past upheavals.The night sky we see today is a result of billions of years of cosmic violence and change.[sciencesources+3]
