Reconstructing the early Milky Way using the most metal-poor stars

Akshara Viswanathan (University of Victoria)

March 30, 2026

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Abstract: Metal-poor stars are among the oldest surviving objects in the Universe, acting as fossil records of the first generations of star formation and the early assembly of galaxies. In this talk, I will show how these rare stars can be used to reconstruct the Milky Way’s formation history–from its chaotic proto-Galactic phase to the emergence of an ordered disc. Leveraging large datasets from the ESA Gaia survey and the CFHT Pristine survey, along with extensive spectroscopic follow-up, I will first discuss the rarity and spatial distribution of very and extremely metal-poor stars in the Galactic halo, and present new catalogues and methods that significantly expand the known population of these objects. I will then highlight the discovery of a surprising population of extremely metal-poor stars on prograde, disc-like orbits, pointing to the existence of a proto-disc in the early Milky Way. By modeling the kinematics of chemically distinct stellar populations, I show that the old disc undergoes a gradual spin-up with increasing metallicity, revealing a continuous transition from a slowly rotating proto-Galaxy to the present-day disc. Zooming out to the outer Galactic halo, I will present new constraints on its metallicity structure, where a large fraction of stars are very metal-poor, and discuss how tidal streams and substructures encode the remnants of past accretion events of the Galaxy. This includes the confirmation of the most metal-poor stellar stream known to date, as well as the discovery of a very metal-poor stellar counterpart to the Magellanic Stream. Together, these results demonstrate how combining chemistry and dynamics in metal-poor stars allows us to piece together the Milky Way’s earliest history, offering new insight into star formation and galaxy assembly in the first billion years of the Universe using our own cosmic backyard.