The Gaia Sausage
The Major Collision That Changed the Milky Way Galaxy

The Sausage Papers

  • Co-formation of the disc and the stellar halo (Published by MNRAS) We describe the local evidence, but also outline explicitly and precisely what sort of event this was, how much debris it could contribute to the inner Milky Way halo. We also analyse Cosmological simulations to pin down the mass and the time of the accretion. and point out that it may have produced the thick disc.
  • Apocenter Pile-Up: Origin of the Stellar Halo Density Break (Accepted to ApJ) We point out that the stellar debris deposited during this event have very similar apo-centers and are naturally responsible for the stellar halo break.
  • The Sausage Globular Clusters (Submitted to ApJ) We demonstrate that this particular merger has brought a large number of Globular Clusters into the MW, and these stand out rather clearly from the rest of the Galactic population
  • The Milky Way Halo in Action Space (Published by ApJ) We point to the evidence for this merger in the distribution of actions and also highlight the existence of the large retrograde spray of debris.
  • The Shards of Omega Centauri (Accepted to MNRAS) We discuss in detail the retrograde debris and provide comparisons to a simple model of a massive merger.

Artist impression of a collision between the Milky Way and a massive dwarf

Credit: V. Belokurov (Cambridge, UK and CCA, New York, US) based on the image by ESO/Juan Carlos Muñoz
Caption: An impression of the encounter between the Milky Way galaxy and the smaller Sausage galaxy. This collision took place about 8 to 10 billion years ago. Although remote in time, the record of this ancient encounter is still preserved in the velocities and chemistry of the stars.

Velocities of the Milky Way and the Sausage stars in Gaia

Credit: V. Belokurov (Cambridge, UK and CCA, New York, US) and Gaia/ESA
Caption:The stars of the Sausage galaxy form a characteristic "Sausage-like" shape in velocity space. This is caused by the strong radial motions of the stars. The vertical axis represents the star's circular motion. The Galactic disc (the disc is the major component of our Galaxy) stars can be seen rotating around the center of the Milky Way with a speed of ~220 km/s. The stars that make up the "sausage" have negligible circular velocity, but they zoom back and forth in the radial direction with a speed that can be almost twice as high, close to ~400 km/s!

Orbits of the Milky Way and the Sausage stars in Gaia

Credit: A. Deason (Durham, UK), SDSS and Gaia/ESA
Caption: Orbits of three Main Sequence (MS) stars inside the Milky Way Galaxy in 3-D. Visualizing the stellar motion in the orbital space has only become possible thanks to Gaia. Before, two out of three components of the stellar velocity were missing (or poorly measured). Here, the green orbit is that of a star in the Galaxy's disc, the blue orbit corresponds to a metal-deficient star in the halo and the red one to a star likely deposited in the Sausage event, with metallicity almost as high as that of the Galactic thick disc. Metal-defficient stars likely came to the Milky Way from small dwarf satellites, while the stars with high metal abundances (green and red) could only be produced in massive galaxies, like the Milky Way or the Sausage progenitor. The red orbit is much more radial compared to both green and the blue one. In other words, most of the red star's motion is to and from the Galactic Centre.

Simulation of the Sausage-like merger

YouTube Link 1


YouTube Link 2

Credit: D. Erkal (Surrey, UK)
Caption: A numerical simulation of an accretion event similar to the Gaia Sausage. The Milky Way is shown in grey-scale while the accreted stars are shown in red. The stars from the accreted galaxy are ripped off and left on extremely radial orbits. The sausage arises when these stars are viewed in velocity space since they have much larger velocities in the radial direction than in any other direction.

An analog of the Sausage merger in the Auriga simulation

Credit: A. Fattahi (ICC, Durham, UK) and the Auriga Collaboration
Caption: This is an analog of the Sausage merger in the Auriga suite of simulations. The three images show the initial stages of the interaction between the young Milky Way and the smaller dwarf.