However thanks to the Gaia spacecraft were learning quite a bit, as a recent research study shows. When the last major stellar merger with the Milky Way happened, the results were published in the Astrophysical Journal and reconstructs.
The globular cluster NGC 2808 is potentially GSEs remnant core. Credit: NASA, ESA, A. Sarajedini, and G. Piotto
The most recent merger seems to have actually been with a small galaxy understood as Gaia-Sausage-Enceladus (GSE), which happened about 8– 10 billion years ago. Did GSE orbit the Milky Way prior to clashing, only to be ripped apart by our galaxys tidal forces, or was the accident more head-on and direct?
To address this concern, the group used Gaia observations of stars in the halo of the Milky Way and integrated that information with a survey of the outer edges of the Milky Way called the H3 Survey. This offered the team a good understanding of the position and motion of halo stars. They then created numerical simulations of various theoretical accidents in between GSE and the Milky Way. They discovered that the design which best fits is a galaxy of about half a billion stars that collided with the Milky Way opposite to our galaxys rotation. That is, a more direct collision. Based upon this design, about half of our galaxys halo stars, and about 20% of the dark matter halo came from this one crash.
The merger with Gaia-Sausage-Enceladus represent most of the Milky Ways mass boost over the previous 10 billion years. It was a big breakfast for our galaxys cosmic day. In the future, the menu are other meals, including those of the little and large Magellanic clouds, both of which are approaching the Milky Way. Then, naturally, there is the grand meal to come, when the Milky Way and Andromeda galaxies clash in about 5 billion years.
Reference: Naidu, Rohan P., et al. “Reconstructing the Last Major Merger of the Milky Way with the H3 Survey.” The Astrophysical Journal 923.1 (2021 ): 92.
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As these superclusters of galaxies end up being more separated due to cosmic growth, they gravitationally turn on each other, and in time the biggest galaxies of the group will consume the smaller sized ones. The Milky Way is one of the larger galaxies in our local group, and so it has consumed smaller sized galaxies in the past. Did GSE orbit the Milky Way before colliding, only to be ripped apart by our galaxys tidal forces, or was the accident more head-on and direct?
They found that the design which finest fits is a galaxy of about half a billion stars that clashed with the Milky Way opposite to our galaxys rotation. Of course, there is the grand meal to come, when the Milky Way and Andromeda galaxies collide in about 5 billion years.
As a result, galaxies tend to form into regional groups. As these superclusters of galaxies become more isolated due to cosmic growth, they gravitationally turn on each other, and in time the biggest galaxies of the group will take in the smaller ones. The Milky Way is one of the larger galaxies in our regional group, and so it has actually consumed smaller sized galaxies in the past.