Loki: An Ancient Chaotic Stellar System Hidden in the Milky Way

A team of astronomers led by Federico Sestito announced the discovery of Loki, an ancient and chaotic stellar system hidden within the Milky Way. Published in Monthly Notices of the Royal Astronomical Society, this finding suggests Loki is the remnant of a massive galaxy that collided with ours during its early formation. The presence of stars with peculiar orbits and an unusually homogeneous chemical composition supports this idea.

The researchers analyzed 20 extremely old stars, characterized by low metallicity, located near the solar neighborhood. Their dynamic behavior stood out: these stars maintain orbits that nearly align with the plane of the Milky Way, with eccentricities between 0.5 and 0.9. Of these, 11 follow prograde orbits, rotating in the same direction as the galaxy, while nine move retrograde, in the opposite direction.

These results suggest that these stars may be the remnants of an ancient system that collided with our galaxy.

Scientists believe this configuration resulted from a violent collision over 10 billion years ago, which dispersed the stars into a chaotic structure. The chemical cohesion of these stars, with narrow abundance ranges, indicates they formed in a closed, coherent environment, similar to a classic dwarf galaxy before being absorbed.

To confirm this, they used high-resolution cosmological simulations called NIHAO-UHD, which show that a small impactor can disperse stars in both prograde and retrograde configurations. The name Loki, from Norse mythology, reflects the chaotic nature of this system. Models estimate Loki’s total baryonic mass at around 1.4 billion solar masses, comparable to the Magellanic Clouds.

This discovery challenges traditional views of the galactic halo's structure, but authors warn that the Milky Way's formation involved multiple accretion events. Loki’s combination of eccentric orbits and chemical homogeneity makes it distinct from other recently identified galactic relics.

The finding redefines our understanding of the Milky Way’s assembly and highlights the importance of upcoming spectroscopic surveys like WEAVE and 4MOST in the southern hemisphere. These instruments could confirm whether Loki is a unique entity or just the tip of a much more turbulent galactic past. Meanwhile, these stars remain silent witnesses to an era when our galaxy was still forming through cosmic collisions and mergers, awaiting modern technology to decode their hidden origin deep within the galactic plane.