Space

Study: Milky Way Flipped 90° After Ancient Galaxy Crash

Does slow halo rotation reveal the Milky Way flipped or unlock clues to how galaxies formed?

Published JUL 22

Study: Milky Way Flipped 90° After Ancient Galaxy Crash

Story

Above: Artist's impression of the Milky Way's merger with Gaia-Enceladus 10 billion years ago. Image credit: ESA; Koppelman, Villalobos & Helmi; NASA/ESA/Hubble

The Facts

Astronomers from Durham University presented research at the Royal Astronomical Society's National Astronomy Meeting in Birmingham on Tuesday suggesting the Milky Way's stellar disc may have shifted orientation by more than 90 degrees following an ancient galactic collision.
The Royal Astronomical Society
The research, led by astronomer Kirill Batrakov, used the Auriga suite of cosmological simulations to analyze 25 Milky Way-like galaxies, finding that those with the slowest rotating stellar halos had each experienced a major head-on merger and a disc flip.
Daily Mail
IFLScience
The collision in question involved Gaia-Sausage-Enceladus, a massive dwarf galaxy that merged with the early Milky Way roughly 10 to 11 billion years ago, leaving billions of stars on highly elongated, sausage-shaped orbits.
Guardian
The Brighter Side News
Data from the European Space Agency's Gaia mission showed that stars in the Milky Way's stellar halo orbit the galactic center at roughly 25 kilometers per second, far slower than the approximately 220 kilometers per second recorded for stars in the spiral disc.
IFLScience
The study also found that the rotation of the Milky Way's stellar halo is closely linked to that of its dark matter halo, suggesting the two may have evolved together as the galaxy absorbed smaller satellite galaxies over billions of years.
The Brighter Side News
ScienceAlert

Sources Split

LEFT

CENTER

RIGHT

ESTAB
CRIT

CENTER

PRO
ESTAB

The Spin

Narrative A

The Milky Way's disc almost certainly flipped more than 90 degrees after a massive head-on collision with the Gaia-Sausage-Enceladus galaxy 10 to 11 billion years ago. Supercomputer simulations of 25 Milky Way-like galaxies show that slow stellar halo rotation is a direct fingerprint of that kind of catastrophic merger and disc reorientation. The Sun itself may have once traveled a completely different path through space. The galaxy's apparent stability today masks a violently transformed past.

The Royal Astronomical Society

Narrative B

Rather than simply rewriting the Milky Way's history, this study could reshape how astronomers investigate galaxy formation. If confirmed, the slow motion of ancient halo stars may preserve a record of the Milky Way's lost orientation, allowing scientists to reconstruct past mergers, trace the evolution of dark matter, and better understand how spiral galaxies — and our own solar system's cosmic environment — came to be.