Some episodes in the Milky Way’s history, however, were so cataclysmic that they are difficult to hide. Scientists have known for some time that the Milky Way’s halo of stars drastically changes in character with distance from the galactic center as revealed by the composition of the stars (their “metallicity”), the stellar motions, and the stellar density. CfA astronomer Federico Marinacci and his colleagues analyzed a suite of computer cosmological simulations and the galaxy interactions in them. In particular they analyzed the history of galaxy halos as they evolved following a merger event. They conclude that six to ten billion years ago the Milky Way merged in a head-on collision with a massive dwarf galaxy containing about one-to-ten billion solar masses in size, and that this collision could produce the character changes in stellar population currently observed in the Milky Way’s stellar halo. Reference: “The Origin of Galactic Metal-Rich Stellar Halo Components with Highly Eccentric Orbits” by Azadeh Fattahi, Vasily Belokurov, Alis J. Deason, Carlos S. Frenk, Facundo A. Gomez, Robert J. J. Grand, Federico Marinacci, Rudiger Pakmor and Volker Springel, 4 April 2019, MNRAS.DOI: 10.1093/mnras/stz159
