Rediscovering the Milky Way with an orbit superposition approach and APOGEE data V. The disc growth and history of star formation

The Milky Way’s (MW’s) star formation history (SFH) offers insight into the chronology of its assembly and the mechanisms driving its structural development. In this study, we present an inference and analysis of the spatially resolved SFH and the MW disc growth. 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. We find that the MW is a typical disc galaxy exhibiting inside-out formation: it was compact at $z > 2$ ($\rm R_{\rm eff} \approx 2$ kpc), had a peak in its star formation rate (SFR) 9–10 Gyr ago, and grew to a present-day size of $\rm R_{\rm eff} \approx 4.3$ kpc. A secondary peak in SFR $\sim 4$ 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. Our work showcases the importance of accounting for radial migration and stellar sample selection function when inferring the SFH and build-up of the MW disc.

💡 Research Summary

This paper presents a comprehensive reconstruction of the Milky Way’s disc growth and star‑formation history (SFH) by combining APOGEE DR17 spectroscopic data with Gaia DR3 astrometry through an orbit‑superposition technique. The authors first adopt the three‑dimensional mass model of Sormani et al. (2022), which includes a realistic bar and disc potential, and fix the bar pattern speed at 37 km s⁻¹ kpc⁻¹. Using this potential they integrate the orbits of ~80,000 APOGEE giant stars, sampling each orbit with 500 phase‑space points. Orbit weights are adjusted so that the summed three‑dimensional density of the weighted orbits matches the analytic stellar density model, thereby correcting for the spatial incompleteness of the APOGEE footprint.

To translate the present‑day stellar mass distribution into a formation‑time mass budget, the authors compute the initial mass of each simple stellar population with Chempy, assuming a Chabrier IMF and including mass loss from SNe Ia, SNe II, and AGB winds. This yields an early‑time mass that is ≈40 % larger than the present mass for populations older than ~2 Gyr, ensuring that the derived SFH reflects the mass actually turned into stars rather than the surviving mass.

Stellar ages are taken from the distmass catalogue (Stone‑Martinez et al. 2024) with typical uncertainties <2 Gyr. Recognising systematic under‑estimates for two problematic groups—young α‑rich (YAR) stars (high‑α but ages <7 Gyr) and metal‑poor stars with poorly converged ages (