In a new paper appearing in The Astrophysical Journal, researchers led by Rohan Naidu combine data from the Gaia mission with follow-up ground-based observations from the MMT in Arizona to document the lineage of stars in the Milky Way’s outer halo. 

Our galaxy, like most large galaxies, has formed through the mergers of smaller systems over time. We can see traces of the most recent mergers in streams of stars that all have the same composition and share orbits that echo back to the trajectory of their host system.

While these stellar streams are fascinating, each of them contributes only a small mass of stars, typically less than 1% of our galaxy’s mass. Hidden in and around all these streams, however, is a population of stars that merged with the Milky Way eight to ten billion years ago and brought with it roughly half the stars in the galactic halo. This diffuse spheroid of stars includes objects orbiting in all orientations, and in this new paper, researchers were able to work out that the object that merged with the galaxy came such that the motions of its objects run counter to the orbits of the stars already here.

You may have noticed I didn’t mention the name of that colliding galaxy. It’s the Gaia-Sausage-Enceledus dwarf galaxy. And yes, that is a ridiculous name. It is referred to in some places as the Sausage or Gaia Sausage galaxy because Gaia found that the remnant stars form a sausage shape with their orbits. Other places, it’s called the Gaia-Enceledus galaxy after Gaia and her Titan son in mythology. And for good measure, many folks just put all three words together for one ridiculously named result.

Folks, please stop letting astronomers name things. Please?

More Information

CFA Harvard | Smithsonian press release

“Reconstructing the Last Major Merger of the Milky Way with the H3 Survey,” Rohan P. Naidu et al., 2021 December 14, The Astrophysical Journal