The Milky Way’s formative years

The Milky Way cannibalized a galaxy one-quarter of its mass ten billion years ago, according to a paper published online this week in Nature Astronomy. The research provides an accurate dating of Milky Way stars.

Galaxies form and evolve hierarchically, with smaller galaxies merging to form bigger ones. The chemical makeup and kinematics of the Milky Way’s stars point to a significant merger in the galaxy’s past, but when this event happened remains under debate.

Carme Gallart and colleagues built an accurate image of the age distribution of stars in the current disk and inner halo of the Milky Way. They find that the majority of stars in the halo of the Milky Way closer to the Sun have ages ranging up to ten billion years old. With the help of cosmological simulations, the authors identified this age limit as the point when the progenitor of the Milky Way merged with one of its then companions, named Gaia - Enceladus.

Knowing the exact ages of stars has also enabled the authors to pinpoint the stars that were present before the merger and those that originated in the assimilated galaxy. Stars that appear redder, due to their higher metal content, trace the original stars formed in the Galaxy pre-merger. The merger not only heated (or puffed up) some of the stars formed in the Galactic disk to be part of its halo but also provided the Milky Way with material to create new stars and give it its current appearance.