An impact with another galaxy around 10 billion years ago filled the Milky Way’s inner surrounding halo with stars and made the galactic disk thicker, reports a paper published in this week’s Nature.

The Milky Way is made up of a number of structures: the central bulge, spiral arms, the disk and the surrounding halo. By looking at the age, chemistry, distribution, and motion of individual stars, it is possible to determine how these components were assembled. Based on models of galactic development, it had previously been suggested that the Milky Way’s halo might have been formed by a limited number of mergers with other galaxies - but the exact nature, timing, and number of these collision events were unknown.

Amina Helmi and colleagues analyse recently released data from the Gaia space telescope - along with the APOGEE stellar survey - and find that the Milky Way’s inner halo is dominated by stars derived from an impact that occurred around 10 billion years ago with a dwarf galaxy that had around a quarter of the (then) mass of the Milky Way. A collision with a galaxy of this size, they conclude, would have also heated the pre-existing galactic disk, forming the thicker disk that exists today.

The authors have named this impacting galaxy ‘Gaia-Enceladus’, after both the Gaia telescope and the Giant Enceladus, the offspring of Gaia in Greek mythology.