Researchers Suggest the Milky Way Flipped Its Disk Billions of Years Ago

• The Milky Way likely underwent a "disk-flip" event billions of years ago due to a massive head-on collision with another galaxy.
• This conclusion stems from Durham University researchers using the Auriga simulation suite to analyze the galaxy's slowly rotating stellar halo.
• ESA's Gaia mission, which mapped billions of stars, provided the foundational data necessary to identify these patterns and the weak 10-20 km/s rotation of the halo.

Scientific Context

• The Milky Way contains two distinct disk structures: a thin, fast-rotating spiral disk containing most stars, and a sparse, slow-rotating thick disk (stellar halo).
• The thick disk consists of older, metal-poor stars absorbed from other galaxies during past mergers.
• Researchers simulated 25 Milky Way-like galaxies over 11 billion years to understand how these structures formed.

The Disk-Flip Hypothesis

• Simulations that successfully recreated the Milky Way’s current slow-rotating halo shared three key traits:
  • Early formation timing.
  • Occurrence of a major merger similar to the known Gaia-Enceladus-Sausage (Gaia Sausage) event.
  • A "disk-flip" event during the galaxy's evolution.
• A disk-flip implies that stellar trajectories, potentially including the Sun’s own path, have undergone massive shifts over time.

Dark Matter Implications

• Findings suggest the Milky Way's dark matter halo likely rotates slowly and may be aligned with the disk in inner regions (r ≲ 20 kpc), becoming vertically oriented in outer regions.
• This twist supports the disk-flip scenario and helps explain the complex history of the galaxy’s formation.