Rediscovering the Milky Way with an orbit superposition approach and APOGEE data

Context . The Milky Way’s (MW’s) star formation history (SFH) offers insights into the chronology of its assembly and the mechanisms driving its structural development. Aims . In this study, we present an inference and analysis of the spatially resolved SFH and the MW disc growth. Methods . Our approach leverages both stellar birth radii estimates and the complete reconstruction of the MW stellar disc using a novel orbit superposition method from APOGEE data, allowing us to trace the orbit-mass weighted SFH based on formation sites, while taking into account stellar mass loss. Results . We find that the MW is a typical disc galaxy exhibiting inside-out formation: it was compact at z > 2 (R eff ≈ 2 kpc), had a peak in its star formation rate (SFR) 9–10 Gyr ago, and grew to a present-day size of R eff ≈ 4.3 kpc. A secondary peak in the SFR ~4 Gyr ago is responsible for the onset of the outer disc, which comprises the metal-poor, low- α population. We find that in situ star formation in the solar neighbourhood started 8–9 Gyr ago. The MW disc is characterised by a negative mean age gradient, as the result of the inside-out growth, with additional flattening induced by stellar radial migration. Conclusions . Our work showcases the importance of accounting for radial migration and the stellar sample selection function when inferring the SFH and build-up of the MW disc.