Exploring the fate of primordial discs in Milky Way-sized galaxies with the GigaEris simulation

Recent observations with JWST and Atacama Large Millimeter/submillimeter Array (ALMA) have unveiled galaxies with regular discs at significantly higher redshifts than previously expected. This appears to be in contrast to our understanding of galaxy evolution at high redshift. Additionally, the stellar populations of the Milky Way suggest that the bulk of the Galactic thin disc formed after, raising questions about the history, evolution, and survivability of primordial discs in Milky Way analogues. Here, we use GigaEris, a state-of-The-Art N-body, hydrodynamical, cosmological 'zoom-in' simulation with a billion particles within the virial radius, to delve into the formation of the early kinematically cold discs (KCDs), defined by their ratio between the mean rotational velocity and the radial velocity dispersion, of a Milky Way-sized galaxy at redshifts. Our analysis reveals a primarily inward migration pattern for disc stars formed at, turning into a mix of inward and outward migration at later times. Stars migrating outwards undergo minimal kinematic heating, and might be identified as part of the thin disc formed at much later epochs. We find that approximately 76 per cent of all stars formed in the KCD at become part of a pseudo-bulge by. This proportion decreases to below 10 per cent for KCD stars formed at. The inward migration of stars born in our KCDs at deviates from the expected inside-out formation scenario of thin discs at lower redshifts. Our results suggest a novel 'two-phase' disc formation process, whereby the early discs transform primarily into the pseudo-bulge within less than a billion years, and the present-day thin disc forms subsequently from higher angular momentum material accreted at later times.