Ancient Upheaval in Andromeda Galaxy

Although neighboring Andromeda Galaxy may look tranquil, scientists are realizing the extent of its tumultuous history. After taking the widest survey yet of faint stars in Andromeda’s neighborhood, a team of astronomers led by Alan McConnachie at the NRC Herzberg Institute of Astrophysics in Victoria, British Columbia, discovered signs of ancient mergers and near-collisions with smaller galaxies, all of which helped shape the Andromeda we see today.

Star map of Andromeda and Triangulum galaxy regions. The two galaxies’ disks are pictured as they normally appear and overlaid on the colored map. Faintly visible around them are "fossils" left from Andromeda's formation, including newly-discovered star streams, dwarf galaxies, and a distortion around Triangulum’s disk indicating Andromeda’s massive gravitational pull. Dashed circles around Andromeda and Triangulum have diameters of about 900,000 light years and 300,000 light years, respectively.
Credit: A. McConnachie/NRC; inlaid image of Triangulum disk courtesy of T. A. Rector and M. Hanna, National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation.

The study reveals the streams of stars and dwarf galaxies left over from past collisions and sheds light on the evolution of large galaxies and the satellites that revolve around them.