Our Milky Way galaxy, often perceived as stable, may have undergone a dramatic "disc flip" in its distant past, according to new research. This colossal cosmic event, likely triggered by a head-on collision with a massive dwarf galaxy, could have reoriented the entire stellar disk by more than 90 degrees, dragging our solar system along with it.
The prevailing theory suggests this "major disc flip" occurred approximately 10 to 11 billion years ago during a violent encounter with a dwarf galaxy known as Gaia–Sausage–Enceladus. While previous research indicated this impact scattered billions of stars into elongated paths, the new study proposes it also caused a fundamental shift in the galaxy's orientation.
"We already know that the Milky Way had a massive head-on collision," said lead author Dr. Kirill Batrakov of Durham University. "So, we think that the Milky Way disc likely flipped in the past."
This revelation emerged from an effort to understand a long-standing puzzle about the Milky Way's stellar halo. The halo, a vast, sparsely populated region surrounding the flat spiral disk, rotates unusually slowly compared to those of other galaxies. The European Space Agency's Gaia mission revealed that stars in the halo can take up to a billion years to orbit the galactic core.
To investigate this, researchers analyzed simulations of 25 Milky Way-like galaxies. They found that galaxies with the slowest-rotating stellar halos shared two common characteristics: a head-on galactic collision and a subsequent disc flip. The Milky Way exhibits both a slow stellar halo and evidence of an ancient collision, making a disc flip a likely scenario.
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Such a flip implies that the stars within the Milky Way, including our own Sun, once moved on trajectories vastly different from their current paths. "Meaning our 'stable' spot in the galaxy might not have been so stable for the Solar System's whole lifetime," Dr. Batrakov noted.
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The study also highlighted the link between the stellar halo's rotation and the galaxy's dark matter halo, the invisible mass that holds the galaxy together. Understanding the origin of the slow-moving stellar halo could offer insights into the nature of dark matter.
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"Finding 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," Dr. Batrakov added. "What excites me the most is that this complex history can be reconstructed just from present-day observations."