Gaia Sausage

Lore & Background

The Gaia Sausage is named for the sausage-shaped pattern of its stars in a plot of radial versus azimuthal velocity, using data from the Gaia Mission. The stars from this merger have highly elongated orbits, with outermost points around 20 kiloparsecs from the Galactic Center at the 'halo break.' These stars were previously seen in Hipparcos data and identified as originating from an accreted galaxy. The name 'Enceladus' refers to the mythological giant buried under Mount Etna, paralleling how this former galaxy was buried in the Milky Way and caused the puffing up of the thick disc.

Reader's Guide

The Gaia Sausage is significant as the last major merger experienced by the Milky Way, fundamentally shaping the galaxy's structure. It contributed at least eight globular clusters, including Messier 2, Messier 56, Messier 75, Messier 79, NGC 1851, NGC 2298, and NGC 5286, as well as NGC 2808, which may be the former core of the Sausage. The merger puffed up the thin disk into a thick disk, while its gas triggered fresh star formation and replenished the thin disk. The debris from the dwarf galaxy provides most of the metal-rich part of the galactic halo. Stars from this merger have extreme orbital eccentricities around 0.9 and typically higher metallicity than other halo stars, with most having [Fe/H] > −1.7 dex.

Did You Know?

• The Gaia Sausage is named for the sausage shape of its stars in a plot of radial versus azimuthal velocity.
• It added at least eight globular clusters and 50 billion solar masses of stars, gas, and dark matter to the Milky Way.
• NGC 2808, a globular-like cluster of the Sausage, contains three generations of stars born within 200 million years.
• The stars from this merger have orbits with extreme eccentricities of about 0.9.