Modern cosmology describes how galaxies evolve. We have to collide and merge it with other systems. In our own Milky Way, we have a clearer view of this build up. The Sagittarius dwarf galaxy is our neighbour and is recently being tidally disrupted. The Large and Small Magellanic Clouds are two other nearby dwarfs and are falling towards us.
Streams of globular clusters encircle the Galaxy. This has marked the effects of prior mergers. The record of ancient mergers is being extracted from the positions and motions of stars in the Milky Way’s stellar halo. The spherical distribution of stars is older than about 10-12 billion years. Andromeda is our nearest large neighbouring galaxy. This is about ten times farther away than these dwarfs.
The Gaia spacecraft was launched to make a precise three-dimensional map of the Milky Way by surveying 1% of its approximately 100 billion stars. Scientists used Gaia results combined with the 6.5m MMT telescope in AZ to piece together the history of the Milky Way’s stars in unprecedented detail. This will determine the nature of the Galaxy’s last merger.
Evidence says a single dwarf galaxy merged with the Milky Way about nearly 10 billion years ago. Gaia-Sausage-Enceladus (GSE) is left of the object today is inferred from the stars in the inner halo. This was done by their stellar motions and compositions. It is uncertain if GSE collided with our galaxy or it orbited the galaxy before merging.
Astronomers modelled Gaia’s measured halo stars with a set of numerical simulations. They have coupled it with a comparison to the stellar ages and compositions. Astronomers say, GSE contained about half a billion stars. These did not orbit the Milky Way but approached it moving in a retrograde direction. Scientists said roughly 50% of the Milky Way’s current stellar halo descend from it. The Milky Way have stars that are about 13 billion years old. As a conclusion, the entire growth of the Milky Way over the past ten billion years can be accounted for.