Since its formation billions of years ago, the Milky Way has merged with 12 galaxies of similar size and three dwarf galaxies, according to a study by astronomers at the University of Heidelberg in Germany.
The researchers arrived at this conclusion by studying globular clusters, dense clusters comprised of hundreds of thousands of stars, in the Milky Way’s halo.
A galaxy’s halo is spherical region of stars, gas, and dust extending beyond its main structure. It typically contains numerous globular clusters and older, metal-poor stars.
To learn more about the Milky Way’s long history, the research team studied 96 globular clusters orbiting the center of the galaxy, measuring the ages and levels of heavy elements in their stars.
Galaxies with large numbers of stars containing heavy elements are typically older than those with mostly metal-poor stars, as the former have had more time to merge with and devour other galaxies in their neighborhoods.
The Milky Way’s globular clusters were found to contain an abundance of metal-rich stars, indicating the galaxy has been merging with other galaxies for as long as 12 billion years.
In addition to having collided and merged with many galaxies of roughly its size, the Milky Way has also devoured at least two dwarf galaxies, the research team concluded, based on their discovery of 25 globular clusters containing metal-poor stars.
Currently, the galaxy is in the process of merging with the Sagittarius dwarf galaxy, one of nine satellite galaxies in orbit around the Milky Way. Completion of the merger will occur over approximately 100 million years, during which the Milky Way’s powerful gravitational pull will tear the much smaller Sagittarius dwarf galaxy apart.
Suspected to harbor some dark matter, the Sagittarius dwarf galaxy survived several previous collisions with our galaxy, the researchers found.
Remnants of galaxies devoured by the Milky Way exist in the form of stellar streams, which the researchers observed with the 570-megapixel Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile.
Based on these stellar streams’ positions and trajectories, the researchers were able to determine they originated in other galaxies that merged with the Milky Way.
A paper on these findings has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.