Mapping the ancient Milky Way through metal-poor stars

Viswanathan explored the Milky Way’s formation and evolution by studying its oldest, most metal-poor stars—those with minimal heavy elements. These stars serve as relics of the Galaxy’s earliest epochs, preserving clues about the events that shaped its structure over billions of years.In her research, Viswanathan examined metal-poor stars across different regions of the Milky Way. She compiled a large catalogue of inner halo stars using Gaia data, enabling her to study star streams—remnants of smaller galaxies or clusters that merged with the Milky Way. Her analysis revealed that these streams, which are faint traces of past mergers, may be influenced by the Galaxy’s dark matter sub-haloes. She investigated the metallicity distribution of the inner halo, finding that very metal-poor (VMP) stars are 60 times rarer than those with slightly higher metallicities. In the outer halo, Viswanathan identified a higher proportion of VMP stars and discovered a stellar counterpart to the Magellanic Stream, providing new insights into interactions between the Milky Way and its satellite galaxies.Using archival Gaia spectra, she created a catalogue of over a thousand VMP stars. With a 77% success rate in finding [Fe/H]<-2.5 stars, she also identified VMP stars with surprising disc-like orbits, and members of the most metal-poor C-19 stream. Finally, her modelling revealed that the transition from the chaotic proto-Galaxy to an organized, rotating disc was slower than previously thought.These findings shed light on the Milky Way’s complex history, offering a deeper understanding of its earliest stages.

Dissertation: https://hdl.handle.net/11370/ac22c64a-82e7-4b70-9e32-92a8a78f9be1