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2w ago

Simulations Suggest Milky Way Disk Flipped After Ancient 'Gaia Sausage' Collision

Researchers say a reorientation could explain the galaxy’s unusually slow stellar halo and points to specific observational tests to confirm the idea.

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Halo 18 is an example of a galaxy that had a head-on collision (see the panel at z=1.2) and had a disc flip (you can see this by comparing the disc orientation at z=1.4 and z=0). This image and Image 2 show how these two exemplary galaxies evolve with time: each panel corresponds to a different time defined by redshift (z). Redshift is a quantity used by astronomers to measure the time in the universe, with z=0 corresponding to the present day, and larger z corresponding to earlier times. Each z has two panels associated with it, which show how the galaxy looks in two planes (like the xy and xz planes in 3D space). Credit Auriga Project
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Overview

  • Durham University researchers presented new simulation results at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham on July 21.
  • Using the Auriga suite, the team tracked 25 Milky Way–like galaxies and found the slowest stellar haloes tended to follow major head‑on mergers and disk reorientations exceeding 90 degrees.
  • They propose the Gaia–Enceladus–Sausage merger about 10–11 billion years ago could have torqued the young Milky Way and slid the disk over a period of roughly 150 million to about 1 billion years.
  • The result is simulation driven and not yet observationally confirmed or fully peer reviewed, so authors call for targeted searches for fossil signatures that would prove a past flip.
  • If true, a disk flip would have changed how most stars moved across the Galaxy, link the stellar halo’s slow rotation to the dark matter halo, and give a new way to reconstruct the Milky Way’s assembly history.