A massive collision in the past of the Milky Way

Our galaxy of the Milky Way probably collided with other galaxies during its lifetime or had other interactions such interactions are common cosmic occurrences. Astronomers can deduce the history of mass accumulation on the Milky Way from a study of debris in the halo of the galaxy left as a tide residue of such episodes. The approach worked particularly well for studies of the most recent events, such as the influence of the Sagittarius dwarf galaxy a few billion years ago that left bands of tidal currents visible on galaxy maps. The adverse effects that these encounters may have on the Milky Way have not, however, been studied as well and the even older events are even less obvious as they are mitigated by the natural movements and evolution of the galaxy. .

Some episodes in the history of the Milky Way, however, have been so cataclysmic that it's hard to hide them. Scientists have known for some time that the halo of stars of the Milky Way is radically changing in character with the distance that separates it from the galactic center, as revealed by the composition of the stars (their "metallicity"), the stellar movements and stellar density.

Astronomer Federico Marinacci and his colleagues analyzed a series of computer-generated cosmological simulations and their interactions with galaxies. In particular, they analyzed the history of galaxy halos as they evolved as a result of a melting event.

They conclude that six to ten billion years ago, the Milky Way fused to create a frontal collision with a gigantic dwarf galaxy containing about one to ten billion solar masses, and that this collision could produce the character changes of the current stellar population observed in the stellar halo of the Milky Way.