🔭 Our Milky Way swallowed another galaxy, Loki, and still bears the trace

At the heart of the Milky Way's disk, a region where stars are generally young and rich in heavy elements, twenty stars stand out for their great age and poverty in metals. This anomaly intrigues astronomers, as they seem to come from elsewhere.

According to a team of researchers led by Federico Sestito, these stars could be the remnants of a dwarf galaxy named "Loki," swallowed by the Milky Way billions of years ago. The study, published in the Monthly Notices of the Royal Astronomical Society, is based on a combination of chemical and orbital data. While previous work had already spotted these stars due to their peculiar motions, adding information about their chemical composition confirmed their common origin.


The oldest stars in the universe, formed shortly after the Big Bang, are composed mainly of hydrogen and helium. By fusing these elements, they produced heavier atoms, enriching subsequent generations. The twenty stars of Loki, being extremely old, contain only traces of iron and other metals. This chemical signature, coupled with their strange orbit, distinguishes them from local stars.

To establish the link, scientists used several techniques: high-resolution spectroscopy, orbital motion analysis, and theoretical simulations. They compared the composition of the stars to that of other stellar populations, such as those in the galactic halo or current dwarf galaxies. Result: the stars of Loki show traces of enrichment by energetic supernovae, hypernovae, and neutron star mergers, but no sign of white dwarf explosions. This indicates a "small and energetic" parent galaxy.

These remnants help understand how the Milky Way formed. The metal-poor stars are rich in information about the earliest moments of galaxy formation. According to Sestito, they reveal the primordial processes that shaped our galaxy and the origin of elements. Finding such groups in the disk is difficult because the region is crowded with young stars. But each discovery refines our picture of the galactic past.

The future looks promising. With future spectroscopic facilities capable of analyzing thousands of stars at once, astronomers hope to uncover other hidden dwarf galaxies. These additional "Loki" will allow a more precise tracing of the building blocks that assembled the Milky Way. As Sestito points out, each ancient star is a precious fossil, carrying the history of our corner of the universe.