Scientists Discover Loki: Ancient Galaxy Swallowed by Milky Way

Astronomers have identified the remnants of an ancient dwarf galaxy, named Loki, which was absorbed by the Milky Way billions of years ago.

The discovery was made through the analysis of a group of 20 stars characterized as “metal-poor.” In astronomical terms, these are stars composed almost entirely of hydrogen and helium, containing remarkably few of the heavier elements that are forged in later generations of stars. These 20 objects were found located within the Milky Way’s galactic disk, the flat, rotating region where the majority of the galaxy’s stars reside.

The location of these stars is considered highly unusual by researchers. Metal-poor stars are typically found in the galaxy’s sparse outer halo rather than the inner disk. Their presence in the disk suggests they did not form there, but were instead brought in from an external source during a galactic merger.

Chemical signatures within the stars provided further evidence of their origin. The research team identified chemical traces associated with supernovas and neutron star mergers. Notably, the stars showed no evidence of white dwarf explosions. Because white dwarfs take billions of years to form, their absence suggests the Loki galaxy burned out and ceased star formation long before such explosions could occur.

The orbital dynamics of the stars further distinguish them from the rest of the Milky Way’s population. Of the 20 identified stars, 11 travel in the same direction as the Milky Way’s rotation, while nine move in the opposite direction. This retrograde motion is a common indicator of stars that originated in a separate galaxy before being captured by the Milky Way’s gravity.

The findings describe the Milky Way’s growth as a process of gradually absorbing smaller neighboring galaxies to achieve its current form. The study was published in the Monthly Notices of the Royal Astronomical Society.