Dr. Khyati Malhan

A key question in modern astrophysics is "How do massive galaxies, like our own Milky Way, form and evolve?" Theories predict that a Milky Way-like galaxy must have formed around 13 Gyrs ago by the coalescence of several massive progenitors. This formation scenario implies that our Galaxy should contain ancient, metal-poor stellar populations concentrated in its inner regions. We examine this prediction using ESA/Gaia's third data release, which provides unprecedented 6D phase-space and metallicity measurements of 33 million stars in our Galaxy. We identify two stellar substructures in the inner Milky Way, that we name ``Shiva'' and ``Shakti''. Both the substructures are metal-poor, on prograde orbits, each possess a stellar mass of M ~ 10^7 msun. Additionally, spectroscopic data from the APOGEE survey reveals unique chemical characteristics of these stars. This analysis supports one of two scenarios: either Shiva and Shakti are results of resonant orbit trapping by the Galaxy's rotating bar, or that they represent two of those massive progenitors that coalesced very early to form the proto-Milky Way. Corresponding ApJ paper: https://arxiv.org/abs/2403.13051