New insights on MW disc history with machine learning, precise ages and Gaia DR3

Abstract

Recent observations of high-redshift galaxies with JWST and ALMA suggest that disc galaxies can form early, while studies of metal-poor stars in disc orbits in the Milky Way (MW) imply a similar early disc formation scenario. Using a chrono-chemo-dynamical analysis of the Gaia -DR3 RVS stars, with high-quality stellar parameters provided by the hybrid-CNN analysis and precise distances and stellar ages from StarHorse, we explore the MW’s early history. We confirm the existence of old metal-poor stars in thin-disc orbits. These metal-poor stars in disc orbits form a readily visible tail of the MW’s old disc, spanning a wide metallicity range from [Fe/H]† < -1.0 to super-solar values. We calculate the vertical velocity dispersion ( σV _z ) for the high- [ α/ Fe] thick disc, finding 35 ± 0.6 km.s ⁻¹ , while the thin disc within the same old-age range has a σ _Vz that is lower by 10 to 15 km.s ⁻¹ . Our findings align the MW with JWST and ALMA observations of thin galactic discs at high redshifts and provide new constraints for reshaping our understanding of galaxy evolution.