Selections from 2024: Possible Proto-Galactic Fragments in the Milky Way

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way

Published March 2024

Main takeaway:

Using data from the Gaia spacecraft, Khyati Malhan and Hans-Walter Rix (Max Planck Institute for Astronomy) discovered two stellar structures within the Milky Way. The two structures, named Shiva and Shakti, each contain more than 10 billion solar masses’ worth of stars and orbit in the inner Milky Way. Though the origins of the two structures are not yet known, current data suggest that they formed more than 12 billion years ago — before the Milky Way’s spiral arms and stellar disk came to be.

Why it’s interesting:

The Gaia spacecraft makes precise measurements of the positions, distances, velocities, and chemical compositions of stars in our galaxy. Using these data, researchers have discovered many new structures and stellar populations in and around our galaxy, such as stellar streams and satellite galaxies. These structures hold important clues to the formation and evolution of our galaxy, revealing a complex history of accretion, assimilation, and migration of gas and stars.

Potential origins of the newfound structures:

Stellar structures within our galaxy can develop in one of two ways: in situ, meaning forming out of gas and stars already present in the Milky Way, or through accretion from outside the galaxy. The orbits and chemical abundances of the stars in Shiva and Shakti tell a conflicting story about the origins of these two structures. The orbits point to an accretion origin, while the chemical abundances suggest in situ formation; this combination of features has never been seen before. Malhan and Rix suggest that these opposing conclusions can be reconciled in one of two ways: 1) the stars of the Shiva and Shakti structures belonged to the Milky Way’s diffuse, extended halo and became trapped in a resonance with the Milky Way’s central bar of stars before migrating to their current positions, or 2) the Shiva and Shakti structures are truly ancient, representing fragments of proto-galaxies that formed before the Milky Way was constructed. While each of these hypotheses has its strengths and neither is fully compatible with the data, Malhan and Rix favor the proto-galactic fragment idea. Luckily, upcoming surveys and instruments will give an even deeper look into the structure of the Milky Way, helping to clarify how our galaxy was assembled.

Citation

Khyati Malhan and Hans-Walter Rix 2024 ApJ 964 104. doi:10.3847/1538-4357/ad1885