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 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.

Erika on X The Brighter Side News PHYS